Missing object.

This is how accountants want to see the system.

Payment related categories shall be setup on Payment Transaction Lines, as they
correspond proper Bank Account.

Specialise can vary, so it is better to have it on lines too.

It is too early, as started or later invoices shall be considered as completed
(from accounting transaction point of view).

So it have to be setup on accounting lines.

It is required by accounting_workflow scripts.

Just add not present yet causalities.

Invoice comes from many causalities which are added continously.

Make invoice multispecialised.

Store not so important categories on movements instead of on invoice itself.