Most items/armor/weapons will be 'usable' by any class. Untrained classes will find using items they are not proficient with to be impractical. Certain non-weapon proficiencies will allow use of particular items (e.g. alchemy allows use of alchemical items.)
A few items will be limited by attribute or alignment (e.g. holy swords and/or divine scrolls)
Most items/armor/weapons will be 'usable' by any class. Untrained classes will find using items they are not proficient with to be impractical. Certain non-weapon proficiencies will allow use of particular items (e.g. alchemy allows use of alchemical items.)
A few items will be limited by attribute or alignment (e.g. holy swords and/or divine scrolls)