Historical Rules · 23 July 2021
Rule 717.4
Official source text from the 23 July 2021 Rules release
Some older creature cards, called leveler cards, have level up abilities that add level counters to them. These are not the same as class level abilities. Level counters do not interact with Class cards, and class levels do not interact with leveler cards. See rule 702.87, “Level Up,” and rule 711, “Leveler Cards.”