A journal mask is the 3-4 letter prefix on a journal entry. That combined with the journal source indicates the purpose of the journal and which system generated it. For example, the Accounts Payable subsystem (source: AP) generates 4 letter journal masks that always begin with 'AP', i.e. APVP, APPY, APCP.
Please see the attached word document for more detailed information on database and other subsystem journal entries. Contact FI-ASD-SUPPORT if you find missing or incorrect information.