>>95887754
Hunter's Mark is just a spell that's on the ranger list, with no class features referencing it. Instead, as a core feature, rangers get a debuff version of Bardic Inspiration to represent them harrying their prey. All the subclasses get various features that interact with this feature, so you can do different things with it depending on what you've specialized in.

Oh, and rangers can maintain concentration on two spells at once starting at 7th level.