The rules for Activating a Magic Item state:
Some magic items allow the user to cast a spell from the item, often by expending charges from it. The spell is cast at the lowest possible spell and caster level, doesn’t expend any of the user’s spell slots, and requires no components unless the item’s description says otherwise.
— Dungeon Master's Guide, p. 141
The phrase was added in a 2018 errata:
[New] Spells (p. 141). In the second sentence, “lowest possible spell level” is now “lowest possible spell and caster level.”
What exactly is caster level referring to in this rule? Further to this, what mechanical purpose does this phrase serve? What behavior is being restricted by this phrase? What ambiguity is being clarified by this phrase?
To be clear, I am not at all interested in any sort of speculation about why the designers included this phrase in the rule. To put it another way, if this phrase were deleted from this rule, what could I do that I couldn't do before? Or, what ambiguity would arise that would no longer be clarified?