Gold-filled jewelry supplies are legally required to be 5% or 1/20 gold by weight.
This 5% is described by the karatage of the gold alloy on the surface.
Most material is 12kt or 14kt gold-filled.
Some will be 12k goldfilled and others will be 14k goldfilled.
There seems to be no rhyme or reason to it.
2mm, 3mm, 4mm are seamed.