Silver soldering uses a much harder solder alloy and higher heat than “soft” soldering in which the solder alloy is a mix of tin and lead. Silver solder - often an alloy of silver, copper, and tin - is used in almost all silver holloware assembly, and you are correct that it makes for a much stronger joint than “soft” soldering.
I’m not aware of the term being associated with how much silver is plated on the base metal. In electroplating, the finished base metal item (often made of copper or brass) is suspended in a tank of chemicals containing some silver and an electrical current is passed through the tank and the silver is plated on the base metal item. The thickness of the plating deposited is very thin, in the range of less than one thousandths of an inch. Multiple layers can be applied to provide a thicker silver plating. The silver that is deposited is pure silver vs. the alloys we see in most solid silver, such as sterling, which is 92.5% silver and the rest another metal, usually copper. Because the silver plating is pure silver, it tends to tarnish slightly differently from sterling and other silver alloys. I notice that silver plate tends to tarnish a purple color whereas sterling tarnishs more a black-ish hue.
There are mnay fancy terms various silver plate makers have devised over the years to try to distinguish their wares from their competitors, such as “A-1”, “quad plate”, “extra plate”, and so on. I’ve not seen any real difference in the quality of the silver plating, other than very old electroplated wares seem to hold up better than more modern stuff, for some reason.