A type of accession identifier indicating an individual unit (or range) for an End Item, usually assigned during the contractual planning stage for any particular production run or series.
Contrast with Serial Number and Line Number: Effectivity represents a contract planning sequence of an order being placed, S/N represents a manufacturing planning sequence, and L/N represents an actual manufacturing sequence. For the most part, these distinctions exist so that the production line is never held up for either contracting or planning in order to avoid severe adverse consequences.