Originally Posted by WineCountryUA

When you purchase a ticket on UA ticket stock 016, that means the segment is "plated" on 016.

What plating restrictions means a carrier will only allow a flight to be sold / plating on their ticket stock or a limited set of ticket stock. Such as, SAS will only allow there TATL segments to be plated on SAS ticket stock and not UA. LH group will only plate intra-europe flights without a transcendentalist segment(s) to be plate on LH Group ticket stocks and not UA. Other carriers will not allow the lowest fares to be plated on other carriers (such as UA) ticket stock. So even allegiance members prevent other alliance members from ticketing certain flights.
Plating is ticketing a segment on a particular ticket stock.
This has nothing to do with codeshares, that's different.

There are many different forms of plating restrictions.
There are many threads in the UA forum on how different partners handle this.
This has been around for a long time (SQ was an early user) but in the last five years or so it has increased.