If the status of a document is 0, then nothing has been printed. Every time a different document type is printed (confirmation, invoice, etc) for the first time, the status number gets incremented by the particular 'binary' number.

For Invoices, this number is 4, meaning any print status less than 4 will not have had the invoice printed. Status 4 means only the invoice was printed and neither of the order specs have been. Status 6 means that the order had the Picking Slip printed (2) and the invoice also printed (4) for a total of 6. Those invoices with statuses of either 4 or 6 have been printed (either to paper, or in a manner as Andrew suggested).

Here is the list of specs and what they add to the print status. Because it is binary addition, you can always tell what particular document(s) have been printed:
+1 for Confirmation
+2 for Picking Slip
+4 for Invoice
+8 for Labels
+16 for Credit Note
+256 if Labels are required.