The first NDC segment, four or five digits, assigned by the FDA to the company that puts its name on the package.
The labeler is whoever is legally responsible for the label — the manufacturer, but equally a repackager, a relabeler or a private-label distributor. Two identical tablets made in the same plant carry different labeler codes if they are sold under different companies' labels.
Labeler codes are assigned in sequence and never reused, so an old code identifies a company that may no longer exist. That is why a discontinued NDC still resolves to a labeler name.