For an asset listing that is grouped by parent asset you have the option of "Restrict Types".
I have read the doco and it doesnt really explain how it works but it seems to me that it works by:
All the assets that are to be included in the asset listing have their parent types checked.
For those assets who's parents are not in the restrict types list their parent's parents (their grandparents i guess) are checked against the restricted types list. If they ARE in the restricted types list then those parents are used, otherwise it goes up the tree again…
Does this sound right? I dont want to use it until I know how its meant to work
Restrict types just tells matrix which asset types are allowed to be parents to grouped items (eg, you have a folder with items underneath, you can tell matrix to only use folders as parents for the group).
It doesn't control the depth at which the asset listing delves into the tree, there are other settings in an asset listing that do that. If you let it delve more than 1 level into the tree, then any folders that exist underneath other asset types also become parents to the groups of assets it holds.