Some people always sign the number 1 with the palm forward. For others, however, the palm faces back (i.e., as 1@Side-PalmBack) for the actual number 1 but the palm faces forward when used as a part of a longer number like 555-1234, as a time (i.e. one o'clock), and in certain other numeral-incorporating constructs.
Like other single-digit ASL numerals, this sign is incorporated into several other signs to indicate a single instance. Several of the derived terms below show that numeral incorporation.