A batch or lot number is a code printed on a cannabis package that identifies the specific production run it came from. Everything made from the same harvest or extraction run shares the same batch number.
Its main job is to connect the product in your hand with the right lab report. When a lab tests a product, the COA lists the batch number tested. If the number on your package matches the number on the COA, you know that report applies to your product. If they do not match, the report may be for a different batch.
Batch numbers also matter for tracking. If there is ever a problem with a batch, such as a recall, the batch number lets producers, regulators and stores identify exactly which products are affected.
For you as a shopper, batch numbers are also handy for keeping notes. If you really like a particular product, write down the batch number along with the strain and brand. The next batch of the same strain may be a little different.
The batch number is usually on a sticker or printed label, often near the packaging date and the QR code. Ask staff if you cannot find it.