What does karat plumb mean?

Karat Plumb means a guarenteed amount. Standard rules in the US says that any item stamped with a karat can have a variance of .5 karat. Example a 14k gold ring can contain between 13.5 karat and 14.5 karat pure gold content. Karat plumb on the other hand means that a ring marked 14kp would have at a minimum 14 karat pure gold content or more.