.ymap

.ymap is compressed file format which is used as map system placement. Since GTA5, this format has XML structure.

CMapData

CMapData is a parent section. It includes file description, flags and entities

<name> - File name definition
<parent> - Subfiles (_strm, _long, etc.) links to the main ymap file.  
<flags> - It's unknown what flags do. Some external formats uses value "2" and other - "0".
<hash_6A675035> - unknown. 

<entities>

This section enumerate objects and sections in ymap file.

<CEntityDef>

     <archetypeName> - Model name
     <flags value/> - Object flags
     <guid value>
     <position x="" y="" z=""> - object coordinates
     <rotation x="" y="" z="" w=""> - object rotation
     <scaleXY value=""/>
     <scaleZ value="1"/>
     <parentIndex value="" /> - Lod Index 
     <lodDist value="" /> - Lod Distance
     <childLodDist value=""/>
     <numChildren value="0"/>
     <priorityLevel>PRI_REQUIRED</priorityLevel>
     <extensions/>
     <ambientOcclusionMultiplier value=""/>
     <artificialAmbientOcclusion value=""/>
     <tintValue value="0"/>

<extensions>

This subsection includes other additional sections.

<CExtensionDefLightEffect>

This section is incomplete. You can help by fixing and expanding it.

<block>

Ignored by the game