| Glancer |
It doesn't have to be a codified rules term if we consider the definition of 'appropriate'.
> adjective: appropriate
> suitable or proper in the circumstances.
You would not be able to cast a 3rd level spell from a staff if you only had 2nd level spell slots available, as you cannot use 2nd level slots to cast a 3rd level spell, and therefore you cannot cast spells 'of the appropriate level'.
Having a 5th level spell slot, for example, allows you to cast any spell you know or have access to of 5th level or lower in that slot (where Jason's clarification comes in). It is 'suitable or proper' (i.e. appropriate) for the purposes of using a staff (in the circumstances), since by virtue of having a 5th level slot you are capable of casting a 3rd level spell.