From the blog "Scrab's Skyrim Tutorial Hoard"

Tags in SLP+ Tags are used to describe and categorize scenes, allowing mod authors (and users) to more easily find scenes that they want to play. In SLP+, to tags are usually passed around a Strings, dividing individual tags by a comma (,). A very simple query could for example look like this: "Facing, Loving". Beyond this point, SLP+ also supports prefixes which can be used to overload query, giving each tag some special behavior. This is something animators won't have to worry about,