The sem tag is described as being for structural equation modeling, but a proportion (my hunch: 25%) of its use is for (econometric) simultaneous equation models.
Is it worth clarifying, or splitting the tag?
In my opinion, splitting seems unnecessary since simultaneous equation models and structural equation models are essentially the same thing -- more precisely, simultaneous equation models are a special case of structural equation models (in the structural equation model world, so-called simultaneous equation models would be called path analysis models). This is according to, among others, Bollen 1987, who claims that this fact is well known. So anyway, to have two separate tags would seem to be splitting hairs, and I think it's actually rather convenient (for our purposes on this site) that they work out to have the same acronym and thus tag.
With that said, I suppose it couldn't hurt to clarify the
sem tag description to point out that some people using this tag will only have the special case of simultaneous equation models in mind.