Potency is a way to compare actions within a single job against each other. Unless an action has autocrit, autodhit, or elemental stance effects (BLM), or is a DoT, a 200p action on JOB does twice as much damage as a 100p action on JOB. DoT potencies will be just very slightly higher than listed if you have any speed at all, but this usually won't make a difference to deciding which action to use based on potencies.

If you want to compare them between jobs, you have to account for traits like Maim & Mend, and multiply the potencies accordingly.