This is what the CRB says:
"Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light. This decrease does not apply to proficiency in wearing the armor. A character wearing mithral full plate must be proficient in wearing heavy armor to avoid adding the armor's check penalty to all his attack rolls and skill checks that involve moving. Spell failure chances for armors and shields made from mithral are decreased by 10%, maximum Dexterity bonuses are increased by 2, and armor check penalties are decreased by 3 (to a minimum of 0)."
So mithral full plate is considered medium for the purposes of what you can do while wearing it, but you need to be proficient in wearing heavy armor to avoid adding the armor's check penalty to all your attack rolls and skill checks that involve moving.
So, for example, a barbarian would still benefit from his Fast Movement ability while wearing mithral full plate, but would suffer non-proficiency penalties unless he had the heavy armor proficiency.
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Mithral armor: What exactly does it mean when it says mithral armor is counted as one category lighter for “other limitations?”
This means that mithral armor allows its wearer to use it when her own class features or special abilities demand her to wear lighter armor; in other words, the character wearing the armor is less limited. For example, a bard can cast spells in mithral breastplate without arcane spell failure, a barbarian can use her fast movement in mithral fullplate, a ranger can use his combat style in mithral fullplate, brawlers, swashbucklers, and gunslingers can keep their nimble bonus in mithral breastplate, rogues keep evasion in mithral breastplate, a brawler can flurry in mithral breastplate, characters without Endurance can sleep in mithral breastplate without becoming fatigued, and so on. It does not change the armor’s actual category, which means that you can still store a creature one size category larger in a hosteling mithral fullplate, and you can’t enhance a mithral breastplate with special abilities that require it to be light armor, like brawling (though you could enhance it with special abilities that require it to be medium armor), and so on.
Mithril is only "treated" as a category lighter for encumbrance. Class abilities that depend on heavy armor should still work (Armor Master - Fighter archetype), and enhancements that can only be placed on heavy armor should still be possible on mithril full plate.