BSID with DZ Entries

Dear Gurus;

BSID will store open items. But when we are opening in se16n with given company code and customer, we can see Doc. Type other than DR.

DG- Credit/Debit Note, ok, But why DZ?

while analysing the entries, partial payment will come as DZ in BSID. But some other entries with Doc. type DZ is also there. DZ means paid doc's,then why it is in BSID, i think it should be there in BSAD? what could be the reason?

Also, in BSAD, why DR?