Each spellcasting class has a set number of spells known. A character may only cast spells known by that class using the spell slots provided by that class as a caster of that class. e.g. A multiclass character with both cleric and sorcerer spells may only cast cure light wounds as a cleric spell, using a cleric spell slot, as cure light wounds is not part of a sorcerer's available spells. Similarly, the character may not cast the spell magic missile as a cleric spell in order to avoid arcane spell failure since it is not part of a cleric's available spells.