Here's some text from the old help files on Custom Fin.:
Account Mask
Enter an account mask to use to selectively include or consolidate information from accounts within the specified range. This field is available only for detailed line types for which you selected Range of Accounts at the Link Type field.
When selecting accounts, you can use the wildcard character (*) to indicate all matching accounts to link to this line. Each wildcard character will mask a single character position. To consolidate accounts within a range, use the consolidate character (@). This feature is useful in consolidating all departmental accounts within a single line. You can use a combination of wildcard and consolidate characters within the same mask.
NOTEThe Consolidate Mask feature using the @ character can be used only on the first line of linkage information. After you have specified a mask using the @ character, you cannot enter additional linkage lines for the current report line.
If the report line is a detail line type, you can use the consolidation character (@) to consolidate matching accounts.
When using a Range linkage type, the account mask or Mask feature is exclusive to a Detail line type.
If your account mask is ###-###-###, enter the following masks to select the accounts indicated.
Enter ***-500-*** to select all accounts within the range that contain 500 as the second segment.
Enter ***-***-010 to select all accounts within the range that end with 010.
Enter ***-***-@@@ to consolidate all accounts within the range based on the first and second segment. Accounts 100-010-001, 100-010-002, 100-010-003, etc., consolidate into one line. Accounts 100-020-001, 100-020-002, 100-020-003, etc., consolidate into another line.
Enter ***-@@@-010 to consolidate all accounts within the range based on the first segment that ends with 010. Accounts 100-001-010, 100-002-010, 100-003-010, etc., consolidate into one line. Accounts 101-001-010, 101-002-010, 101-003-010, etc., consolidate into another line. Account 100-001-020 are not included in the consolidation.