I wouldn't be too interested to use a link on multiple chains. New link on old chain is a waste of a link & it would unfairly assume more than its fair share of the load based on wear disparity. Old link on new chain? Similar thoughts. If it wasn't worn out, why'd you replace it?

When the chain wears, the link wears with it, presumably at the same (or similar) rate as the rest of the chain. Reusing the same link on the same chain, however...sure. I would call it a "matched set." But I wouldn't be keen to make/break the link gratuitiously. When they snap together/apart easily...say after 4 or 5 uses, I consider, but don't necessarily buy, a new link. The chain is usually worn enough by that time to replace the whole kit & kaboodle by soon anyhow.