.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 hash
     <flags value/> - Flag object
     <guid value>
     <position x="" y="" z=""> - object coordinates
     <rotation x="" y="" z="" w=""> - object rotation
     <hash_9CA32637> - unknown
     <hash_10FB7C42> - unknown
     <parentIndex value="" /> - Lod Index (?)
     <lodDist value="" /> - Lod Distance
     <hash_CA974BCD> - unknown
     <hash_6C8F1715> - unknown
     <hash_A687AC89> - unknown
     <hash_2691F019> - unknown

<extensions>

This subsection includes other additional sections.

<CExtensionDefLightEffect>

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