The rules.bob files contain the commands that BOB uses to process files and to make mod packs.
Currently there is this rule in working_data root:
<Files> = -*.pack PackFile = <retail>/data/mod.pack PackType = mod
This means it packs everything except packs to mod.pack.
If this rule is moved to a subdirectory and add the BasePath property then the contents f that folder can be redirected to another mod pack:
BasePath = / PackFile = <retail>/data/another_mod.pack PackType = mod
The <Files> filter is not necessary here because probably no one has a pack file in a subdirectory.
The <Files> filter can be used for refining what goes where.
<Files> = *.dds, -*_normal.dds, text_files/*.txt BasePath = / PackFile = <retail>/data/another_mod.pack PackType = mod