Hello Andrewas, welcome to Ironworks, I think you should post this in the Baldur's Gate II: Throne of Bhaal forum instead, however I will try to answer as well as I can:
A thief with this can use ALL potions, wands, scrolls and things that can be put in the quickslots, on things which do not fit in the quickslots, I think he can use anything that is not prohibited because of his alignment or stats, I'm not sure about anything other than the quickslot things though.
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Worshipper of Tiax.
Wielder of the ancient Lawyer spells.
Member of the HADB clan.
Ally of the Fluffy Queen and the Dreaded Red Fluffy
General of the highest class and Leader of the Legal Department in the IW Peacekeeping Force.
ORT member, Official Demander of Stuff.
Sorcerer of Lady Lioness' Court.
"If Silence is golden, why does everyone talk so much?"