Shelf life is a technical property: the period over which the product stays within specification under defined storage conditions, established by the producer through testing and modelling. The best before date is the labelled expression of it, and the two are rarely the same number by the time goods change hands.
Why it works this way
Shelf life is set at production, from stability data, storage temperature, packaging barrier and, for dried products, water activity. It is a statement about the product. The best before date is a statement to the consumer, and it has to be defensible for the whole distribution chain, so producers build in margin.
Buyers then apply a remaining-life rule. A retailer commonly requires that goods arrive with a stated fraction of total shelf life left, often two thirds or three quarters. Anything that arrives with less is technically in specification and commercially unacceptable.
What follows in practice
Specify all three numbers separately in the contract: total shelf life under named storage conditions, the date format and marking on the pack, and the minimum remaining life on arrival. Leaving the third one out is the most common cause of a compliant delivery being rejected.
For frozen goods, remember that the date assumes an unbroken chain. A logger showing an excursion does not shorten the printed date, but it does put the underlying shelf life in question. Moisture, water activity and shelf life sets out how the technical figure is established.