In the recent book Xanathar's Guide to Everything, there are some subclasses/archetypes added for Rangers, among them the Monster Slayer (p. 43), which gives some specific helpful features.
At 3rd level, you gain the ability to peer at a creature and magically discern how best to hurt it. As an action, choose one creature you can see within 60 feet of you. You immediately learn whether the creature has any damage immunities, resistances, or vulnerabilities and what they are. If the creature is hidden from divination magic, you sense that it has no damage immunities, resistances, or vulnerabilities.
You can use this feature a number of times equal to your Wisdom modifier (minimum of once). You regain all expended uses of it when you finish a long rest.
The limitations on use are clearly spelled out.
Starting at 3rd level, you can focus your ire on one foe, increasing the harm you inflict on it. As a bonus action, you designate one creature you can see within 60 feet of you as the target of this feature. The first time each turn that you hit that target with a weapon attack, it takes an extra 1d6 damage from the weapon.
This benefit lasts until you finish a short or long rest. It ends early if you designate a different creature.
So from this, I would interpret it as it being able to switch from creature to creature at will, but there is no guidance as to what happens if the creature under the mark dies, or if you can carry it (so to speak) from encounter to encounter.
Is there a limitation on the use of Slayer's Prey in the wording that I am missing? Or is this a basically a multiple use feature? (i.e., you can designate any creature as a bonus action, with no limitations on uses per day)?
The extra d6 seems a little overpowering at lower levels, and somewhat meh at higher levels.
FWIW, the GM basically allowed it as multi-use for the session, and was going to also try to find clarification.