Sequence numbers are used to automatically generate article numbers, invoice numbers, person numbers, etc. Wherever sequence numbers are used, e.g. in the invoice dialog, they can also be configured. The following dialog appears which can also be called from Settings:

Add / Edit / Copy [more]
Hereby you can create a new sequence number resp. edit or copy an existing one.

Delete
Hereby you can delete one or multiple sequence numbers. Sequence numbers already in use cannot be deleted.

Default sequence numbers [more]
Here you configure which sequence numbers are used by default, e.g. for book entries or articles.