I'm writing a packing procedure report. And I don't know what the open part at the back of a pack should be called. Does anyone know this?
If the opening in the back of the pack is one that can be opened and closed, especially if the packing is made of plastic, it could be called a ziplock.
having an interlocking groove and ridge that form a tight seal when pressed together a ziplock plastic bag
If it's a permanent fixture that must be broken to open and cannot be refastened, I would call it a seal.
2 a : something that secures (as a wax seal on a document)
b : a closure that must be broken to be opened and that thus reveals tampering
I would take a look at google image searches of these terms and see if they fit with what you're referring to.