This asset stores data related to a lightmapping bake. This includes:
Just after a scene loads, Unity will read this data asset and apply the settings from it to restore the lightmapping. This happens in the player and at runtime.
This asset is a standard Unity object. The format and data layout can be found via reflection of serialized properties.
The community com.newblood.lightdata package can be used to edit these assets, and contains the most up-to-date data format.