The FAQ in the manual contains a list of possible values in accountlines.accounttype: https://koha-community.org/manual/19.05/en/html/faq.html#fines-table Bug 22521 changes those values: https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=22521 I'll try to spend some time nailing down what the manual should say and put that here.
Submitted a patch to the manual: https://gitlab.com/koha-community/koha-manual/merge_requests/322 Please also see this change already done for newer manuals: https://gitlab.com/kfischer1/koha-manual/commit/296559cdb5b251a269f0e74e2ef8bb1e31a0212b