An account specification is a compilation of invoices containing the records from receivables and the general ledger.
It can be printed during invoicing or afterwards. This is set in 'Settings - Customer Order Invoicing' (CRS722).
The information contained in account specifications is also contained in 'CO Invoice. Display' (OIS350). Account specifications are printed using 'Account Specification. Print' (OIS690) where one or more specifications can be printed at the same time.