If you use manufacturing with locations and bins, when you create a production order, the Bin Code on the header is filled from the "From-Production Bin Code" field on the Location Card.

However, this only happens if the Item's Replenishment System is Production Order.

If it's Purchase, then the Bin code is blanked.

It is quite common, that companies choose how to replenish an item at the time of planning, so an items that you by default produce yourself may be purchased at a vendor, or an item you normally purchase you choose to produce yourself.

It often depends on the production capacity at the time of order.