Some direct answers:
1-It means: transform the .dat physical track declaration into .gp4 physical objects (track tarmac, banks; kerbs, rails are objects).
2-Quickly create a physical track corresponding to a .dat track
3-the .dat is physical but its conversion is graphical so at the end you have both.
4-open .gp4 and .dat at the same time in 3ded, go to any scene, go to object level and hit the add button, choose "track from dat" or/and "pitlane from dat".
5-Mapping is automatically done, you just have to choose a texture to apply to each object.
6-To quickly create a custom track.
Hope it answers most of you queries