i filtered on currhist but did not find any
>just to help me understand, why was this done this way.
>I never dealt with SBT until now when we abandon it for the states own accounting system (peoplesoft - argh!)

armast contains all open (not completely paid or cancelled) invoices and some that have been paid or cancelled.
They won't leave armast until:
a: the invoice is fully paid or cancelled
and then
b: the period is closed.

When the period is closed, however, all invoices whether paid or not, that have been created since the last closing are added to arymst.
Therefore, prior period invoices that have not been paid or cancelled are redundant. An X in currhist identifies the redundant invoices in armast.

Lousy design by today's standards, for sure, but back when SBT first started the world was different and all this made perfect sense.