BIDS uses custom user defined labels in several situations (naming of participants, sessions, acquisition schemes etc.) Labels are strings and MUST only consist of letters (lower or upper case) and/or numbers. If numbers are used we RECOMMEND zero padding (“01” instead of “1” if you have more than nine subjects) to make alphabetical sorting more intuitive. Please note that sub- prefix is not part of the subject label, but must be included in file names (similarly to other key names).