According to this page: http://burnanenergyjournal.com/forms-of-energy-motion-heat-light-sound-2/
Potential Energy is any type of stored energy. It can be chemical, nuclear, gravitational, or mechanical.
I may be mistaken, but it seems to me that a super-cooled liquid stores potential energy. This is used in sodium acetate heat packs which release this potential energy as heat when the gel they contain transitions from liquid to solid.
This definitely isn't nuclear or gravitational.
I consider mechanical as being the energy stored in a spring when it's (e.g.) compressed, and I consider chemical to be the energy that's release when (e.g.) sodium metal reacts directly with water.
I don't believe it's either of these, chemical is close, but there is no chemical change in a heat pack: it's sodium acetate and water going in, and sodium acetate and water coming out.