How do batch and expiry tracking work, and what is FEFO?

On a trade that tracks batches (pharmacy, wholesale and others you switch it on for), every purchase opens a lot with its own quantity and expiry date. Billing picks the nearest-expiry lot first automatically (FEFO), and an expired lot cannot be sold. A lot with no expiry date printed on it — hardware, cosmetics — is simply left undated; it still holds stock, it just never appears in the expiry report.