Intermediate Classes can be taken by units at least LV10 or above, and will require Intermediate Seals. The special male-only class, Dark Mage, will require a Dark Seal that can be obtained by defeating the Death Knight.
Aside from Stat changes, there are intermediate classes that has their own Class Abilities. You can see them in the bottom of the Stat Changes page. Press X to display the details of the class abilities available for a particular class. This is the class tier where mastery over several Skills are required to get certified to a certain class.
Some class Abilities can be learned permanently by mastering a specific class. That way, you can equip/ unequip it like a normal Ability, even if you switch classes. There are certain class abilities however that can’t be learned even after mastering them. (like the Thief’s Locktouch or the Cavalier’s and Pegasus Knight’s Canto)
This class is only available the Claude, Dimitri, or Edelgard.