The Ranger's Primeval Awareness feature states:
Beginning at 3rd level, you can use your action and expend one ranger spell slot to focus your awareness on the region around you […]
However, the section on Multiclass Spellcasting states:
Spell slots You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, and half your levels (rounded down) in the paladin and ranger classes. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table […]
In this case how does one determine which of their spell slots count as a "ranger spell slot"?
There was a similar case with the Paladin's Divine Smite feature which was cleared up by an errata: "If I multiclass into Paladin, can I use up any class's spell slots for Divine Smite?"
Please note that I am asking solely about other classes with the Spellcasting feature and not multiclassing into Warlock, which I ask about here.
Best Answer
Three options: all spell slots count, no spell slots count, or non-fungible spell slots can be marked out for restricted use when necessary.
There are three options for resolving this, given what's currently known about this precise issue:
My personal ruling would follow Allan Mills' answer. It's easy to implement and follows standard rules for spellcasting features.
But if you really want to mark out which spell slots are attributable to a character having a certain number of levels in the Ranger class, you can derive it from the steps used to calculate how many multiclass spell slots should be available:
Calculate which row in the multiclass spellcasting table your character has, including their Ranger levels.
Calculate which row in the multiclass spellcasting table your character would have, if they had no levels in Ranger (or any other class with the spellcasting feature)
The "extra" spell slots resulting from the levels in Ranger can be considered "Ranger slots", as without the levels in Ranger they would not be available to the character. These should be marked out separately as extra slots within a level, such as the extra level 1 slot gained from moving from the level 1 row to the level 2 row on the multiclass spellcaster table, or as newly available slots from higher levels.
For most purposes spell slots are fungible between classes for multi-classed spellcasters (it doesn't matter which class "contributed" the slot you use to cast it, since very few spells are restricted by type of slot). Because of this it makes sense to treat only the last X slots available as having been contributed by the class for which there is a spell with a restriction, where X is the number of "extra" slots contributed.
If you have at least X spell slots available at the level you want to cast the spell with the restriction, you can cast the restricted spell and then mark the spell slot "contributed" by that class as having been used and no longer available to cast the restricted spell.
Whenever spell slots are recovered, you can apply similar logic. If using some means which allows you to recover slots of a specific class, it may be reasonable to rule that your "extra" slots are restored (which can then be used for a restricted spell or an unrestricted one). Events which refresh all spell slots restore everything, "extra" slot or no.
Let's look at an example of the simplest case:
A character is Ranger 3/Wizard 1. That equates to one level contributed by the Wizard class, and one level contributed by the Ranger class (1*W + 0.5*R) = (1*1 + 0.5*3) = 2.
Without the levels in Ranger, this character's multi-classed spell slots would fall on row 1 and would have two 1st level spell slots. With the levels in Ranger, this character would fall on row 2 of the table and would have three 1st level slots. The Ranger class levels are contributing one 1st level slot.
So as long as this character has at least one 1st level slot left, and has not already used a spell requiring a slot restricted to the Ranger class, they would have one 1st level "Ranger" spell slot available to use. This character would be able to cast Primeval Awareness once, after which they would need to restore the "Ranger" spell slots in order to cast it again.
Now let's look at a slightly more complicated case:
A character is Ranger 3/Wizard 2. The formula for spell slots on the multi-classed spellcasting table would then be (1*W + 0.5*R) = (1*2 + 0.5*3) = 3.
Without the Ranger levels, this character would be on the second row of the table and would have three 1st level spell slots. With the Ranger levels this character would be on the third row, granting four 1st level slots and two 2nd level slots. The Ranger class levels' contribution is one additional 1st level slot and both of the 2nd level slots.
So for this character to still have a "Ranger" spell slot, they would need to have either of their 2nd level slots or at least one 1st level slot available in order to cast Primeval Awareness. And since all three of those slots were "contributed" by the Ranger levels, this character would be able to cast Primeval Awareness up to three times before needing to restores spell slots through means which do not exclude Rangers.
And there you go. An official ruling will almost certainly not include this convoluted scheme, and so would make it irrelevant, but until then I don't see any other options that bud directly off of RAW:
I know what I would choose, but some people may be attracted to bookkeeping horror or acute drawbacks to multi-classing.