Custom Filter-Depend

Hi all,

i am very confusing. i have couple of questions for you.

1. what is filter-depend in se18?

2. there is a filter attach with BBP_DOC_CHECK_BADI and BBP_DOC_CHECK_BADI, how does it work? because filter is a data element then what the logic behind and how it gets trigger

3. is it possible to create custom Filter in se18?

please give me brief explanation.

thanks,

john.