Spar varnish is named "spar" because of the spars on a boat signifying that it was an outdoor varnish suitable for boats.

Almost all spar varnishes are polyurethane varnishes. Polyurethane means it uses artificial plastic compounds and also sounds impressive. Very few varnishes are real "old school" pure varnishes made for ingredients from pine trees. Ephiphanes spar varnish is one of them. I used it and it's great. Takes a little longer to dry but a gloss finish that has outlasted any poly varnish (spar) that I ever used. Not as hard as Polyurethanes. That's why polyurethanes were invented - actually for floor coatings.

So, system three polyurethane varnish is an exterior or "spar" varnish - probably as good as anything out there. As Dan wrote, the WRLPU stuff is a two part urethane coating and I would stay away from that. Probably super stuff for a bar top.