Why does the process shipment separate the product??

There is such a thing as a batch of product. You accepted not 1000 pieces of goods in one batch, but several times by X pieces.
The write-off occurs in batches, which means that it takes the oldest balance, writes off everything that is there, then the next one, and so on until the balance runs out.
For example:
1. Receipt for the 1st day of 10 pcs.
2. Receipt for the 2nd day 10 pcs.
3. Selling 12 items of goods takes 10 items from receipt on the first day, 2 items from receipt on the second day.