Interleaved 2 of 5, often written ITF, is a numeric-only linear barcode that encodes two digits in the space of one by interleaving them: odd-position digits are carried in the bars, even-position digits in the spaces between them. That makes it one of the densest linear formats, which is why it took hold on shipping cartons and warehouse labels where print area is scarce.
Because digits are paired, the data must have an even number of them; an odd-length value is padded with a leading zero. ITF-14, the fourteen-digit variant used for trade units on outer cases, is Interleaved 2 of 5 with a fixed length and a check digit fixed by the standard.
Its weakness is partial reads. Without a check digit or a bearer bar around the symbol, a scanner sweeping across only part of the barcode can return a shorter, valid-looking number. Retail implementations handle this with fixed length, a check digit and quiet-zone discipline, which is exactly what ITF-14 codifies.
For loyalty this format appears mainly through legacy: a member number inherited from a logistics or ticketing system already expressed in ITF. Google Wallet renders ITF-14 on passes; Apple Wallet's formats are QR, PDF417, Aztec and Code 128, so an Apple pass carries the same digits in one of those. See QR and barcode identification for choosing a format per estate.