There are two suffixes that may show beside document numbers on the Card List: an "M", and/or a "D". These suffixes are explained below.
Suffix M
An "M" will show beside a document number when the number has been manually modified (and is no longer following the auto-numbering format set on the project - see article: Auto-Numbering Format). If this was done in error, typing [clear] in the grid (or in the CSV) will reset the card back to an auto-generated value (see article for more details: Update Document Numbers).
Suffix D
A "D" will show beside a document number when the number is a duplicate to another card on the project. To quickly review if there are any duplicate document numbers on a project, filter the column to "Show duplicates only".
If both "M" and "D" are shown as a suffix, it means the document number was both manually modified and is also a duplicate to another card on the project.
Note that only cards which are marked as required will be considered when identifying duplicates. In addition, cards which are included as part of merged sets will not be considered when identifying duplicates.