Intersection group number is an AC8 feature that controls which layers clean up to each other. If the numbers match, they do, if not, they don't.
The numbers work even when layers are hidden. So, for plan, we give plan walls group number 1 and high (and low) walls group number 2; otherwise, there would be a missing line where visible walls meet invisible high walls.
It also works for corners in 3D, where we want all the walls to clean up, so plan walls and high walls should both have group number 1. Therefore, while working in section/elevation LCs, your plans will have gaps. Ignore them.
In addition projects, we also need to account for demolished elements. Demo walls have group number 3, which keeps them from interfering with both plan and high walls.
The group number is saved for each layer with the layer combination. To summarize:
All Plans:
1: A WALL INT & EXT, S Wall
2: Wall Hi & Lo, S Wall3
3: (Additions only) All Demo walls.
All model views, including elevations & sections:
1: All walls
3: (Additions only) All Demo walls.
See Also:
Archicad User Guide 1 pg. 39
(Reference given with extreme prejudice. This page is full of errors and half-truths. Use at your own risk.)