The diameter of the metal that each link is made of. Hmm trying not be be ambiguous here - hope that that sentence makes sense!
If it doesn't: imagine taking a link out of the chain, cutting through it and bending it into a rod. Now measure the diameter of the rod that you have made. Hope that makes it clear which diameter you are measuring!
you measure the diameter of the link material with a caliper. Within the diameter you get standard chains, long link chains and short link chains. and that you measure on the inside of the link I hink. that is the length of the inside of the link