I have always seen magic the gathering like this: 

The library/ deck is all the spells known to a planeswalker. mill cards often discuss forgetting or the destruction of knowledge. So if you are hit with a mill spell you forget temporarily that you knew those spells. 

The hand is all the spells currently prepped for use. You have spent time to "draw" the spell out of your memories and are ready to cast them at the right moment. Discard spells destroy the prepared spells in the hands while draw spells allow you to ready another spell for use. 

The graveyard is for spells and creatures that have already been used. 

I do not know if this is helpful but these are my thoughts. Magic the Gathering is diverse in its magicks and while some of its mechanics do not translate into a rolepay easy, they can still be considers.