There are a number of questions on this site about second-order logic (admittedly I may have asked a few more of them than is reasonable). For example, searching "second-order logic" (in quotes) currently returns 152 hits, and skimming through them most do indeed seem to be about $\mathsf{SOL}$. Math.stackexchange does have a tag for this topic, and I'd like to add one here too, but I want to get some feedback first:

Does it seem reasonable to have a "second-order-logic" tag?

Here's a possible tag description:

For questions about second-order logic, or more generally any logical system with quantification over sets/relations/functions as well as individual objects.