Walled cities like Solitude, Riften, Whiterun, Windhelm etc. are child worldspaces that are embedded into the Tamriel parent worldspace that they use for LOD. Note that Markarth does not use Tamriel for LOD, it has its own very simplified and barren worldspace with LOD, but copying of references is also possible.
The few cells in the child worldpace making up a city replace the cells with the same coordinates in the parent worldspace, while everything beyond the active cells is just the LOD from the parent worldspace.
The vanilla game deliberately omits references in the child worldspace outside the walls to lessen resource usage. It can make for barren views when peaking over the walls.
The barren views can become even more apparent when increasing the active cell area (uGridsToLoad) or the large reference grid (uLargeRefLODGridSize). In addition not all mods add references in both the child and parent worldspace to ensure matching visuals. There are (also vanilla) cases where it is not really possible to perfectly match things because of objects being moved or they are clipping with other things hidden from normal view angles when inside a city but would become apparent when they would exist in the parent worldspace. In particular Windhelm is really problematic in this regard. If "perfect" matches and visuals for the cities are desired, consider using Open/Exterior Cities mods instead.
DynDOLOD can make copies of references which have been added to a child worldspace only to the parent worldspace and vice versa to ensure better visual continuity. Note that in particular copying references from a parent worldspace to a child worldspace can result in noticeable higher performance requirements. In case the exteriors of Whiterun and Solitude are still mostly vanilla, the old static Whiterun Exterior and Solitude Exterior patches from DynDOLOD Patches can be installed instead. Do not check the Parent > child checkbox in order to use the patches.
The Child > parent option copies references that could have LOD from the child worldspaces to the parent worldspace. The copied reference usually uses the LOD model similar to the vanilla game.
The Parent > child option copies references that have LOD from the parent worldspace to the child worldspaces. This replaces the former Whiterun Exterior and Solitude Exterior patches installed with DynDOLOD Resources, which manually placed copies of selected vanilla references. The old static patches should not be used anymore to have an exact replica of the exteriors, including modifications made by mods.
If there are any issues, post a problem report to the official DynDOLOD support forum. Especially when encountering duplicate or out of place objects with mods.
Generation Settings
To globally control the preset for copying references from child worldspaces to parent worldspaces, use the Child > parent drop down in the advanced mode settings. See below for how the presets are defined for each child worldspace individually. For example, the medium setting will also make copies of most tree types, while the low setting will not. The low preset is the former default setting.
To globally enable or disable copying of references from parent worldspaces to child worldspaces, use the Parent > child checkbox in the advanced mode settings. See below how to enable or disable copying of references to specific child worldspaces individually.
Individual Child Worldspace Configuration
For a child worldspace to actually be considered for copying of references, a configuration file with the worldspace Editor ID needs to exist at ..\DynDOLOD\Edit Scripts\DynDOLOD\Configs\DynDOLOD_[GAME MODE]_childworld_[WORLDSPACE].ini. For example DynDOLOD_SSE_childworld_WhiterunWorld.ini.
NoScanIfPluginExists - do not do anything if one of the listed plugins exists regardless of any other settings.
Bounds - defines the area for which to generate LOD for trees in object LOD. See engine bugs below.
NoDynamicLOD - this setting disables showing any dynamic LOD from the parent worldspace when the player is in the child worldspace.
Settings for Child > Parent
ScanChild - set to 1 to copy references from the child worldspace to the parent worldspace. While this can be disabled for specific child worldspace, current reference rules expect this to be enabled for Whiterun and Solitude. Otherwise certain vanilla buildings will be missing in the Tamriel parent worldspace.
IgnoreChildEDID[Low|Medium|High] - do not copy any references from the child worldspace to the parent worldspace if the reference uses a base record with a matching Editor ID. Which low, medium, high setting is used depends on the Child > parent drop down selection in the advanced mode settings.
Settings for Parent > Child
ScanParent - set to 1 to copy references that have LOD from the parent worldspace to the child worldspace.
NoCellsWithNAVM - set to 1 to not copy references from the parent worldspace if the child worldspace cell contains a nav mesh. Use to only copy reference that are really outside the walled cities.
IgnoreParentEDID - do not copy any references from the parent worldspace to the child worldspace if the reference uses a base record with a matching Editor ID.
BoundsIgnore - defines an area of cells to ignore for parent worldspace to child worldspace copying.
Ignore All References Added by a Mod
See the explanations How to ignore all references of a child worldspace for copying to parent worldspace and LOD.
Specific Reference Matching
The patching process tries to match references in the parent and child worldspace based on their used base record, position and rotation.
The log file ..\DynDOLOD\Logs\DynDOLOD_[GAME MODE]_ChildworldMatches.txt reports the found matches.
Its content is based on the already known matches listed in ..\DynDOLOD\Edit Scripts\DynDOLOD\Configs\DynDOLOD_[GAME MODE]_ChildworldMatches.txt and matches that have been found for plugins in the current load order.
The first reference is typically placed in the child worldspace, while the second reference is typically placed in parent worldspace.
If the same reference is listed for both, it means it does not have a match in the other worldspace but should also be ignored for copying.
In case a mod requires manual additions to specifically match references or to ignore them for copying, make a post on the official DynDOLOD support forum so the data can be added for all users in a future update.
Engine Bugs
Standard tree LOD does not unload in the child worldspace - both full model and billboard show at the same time. DynDOLOD fixes this by generating LOD for trees in object LOD for cells with coordinates of the walled cities.
If a cell in a child worldspace has no LAND record, the height and texture data of the LAND record defined in the parent worldspace is used instead. For that to always work properly, a reference needs to be present in that cell. Otherwise the full terrain might be missing and the lowered terrain LOD can be seen instead. DynDOLOD fixes this vanilla bug by adding references to cells as needed.
If a cell in a child worldspace has no large references defined while there are large references for the cell in the parent worldspace, the object LOD for the large references does not unload. This happens to visually work mostly for the vanilla game but becomes a visible problem when using grass LOD with mode 2 and sometimes with mods. DynDOLOD fixes this when using large references bugs workarounds and checking Parent > child in the advanced options by making copies of references that have LOD in parent worldspace to the child worldspace.
While not an engine bug, the vanilla game places large occlusion planes or boxes around Solitude that causes objects and LOD to not render from high positions on certain walls or buildings (which are hard to reach normally). Install the Solitude Occlusion Planes patch included in DynDOLOD Resources to address this and accompanies the Parent > child reference copies for Solitude.