Generally, you can use nicotine salts anywhere you use freebase nicotine. Nicotine strength in salts vs freebase is roughly equivalent although the effect and duration differ a bit -- at equal density (mg nic per ml solution), salts seem to hit a little harder, kick in a little faster, wear off a little sooner. Throat hit for salts is about half of what you get from the same strength of freebase so you can go much higher in strength with salts. Salts may gunk your coil a bit faster than freebase.
I use salts and freebase interchangeably in all of my RTA's, depending on which bottle is closest at hand when I'm refilling.
Finally, you'll notice that my descriptions are all subjective. These are all generalizations and YMMV.
Ken