SolvaSet is like MicroSol, for softening the decal after it is in place so the decal snugs down around the model's surface. OK to use this softening agent after the decal has dried.
In my experience, SolvaSet was stronger than MicroSol, and I used SolvaSet if the MicroSol didn't work very well. However, the bottle of SolvaSet that I bought most recently seems less effective than what I remember. The smell seems a little different, indicating a newer formula, like everything else.
Chemistry majors among us may have stories about their use of vinegar and something else to do the deed for a much better price. I am not a chemist.
Also, Testors, in their Modelmaster line which is going away, had a decal setting solution. I think I have used it a few times - don't remember how it compared to MicroSol/Solvaset.
I don't use many decals.