Historical Rules · 5 October 2018
Rule 306.5d
Official source text from the 5 October 2018 Rules release
Each planeswalker has a number of loyalty abilities, which are activated abilities with loyalty symbols in their costs. Loyalty abilities follow special rules: A player may activate a loyalty ability of a permanent they control any time they have priority and the stack is empty during a main phase of their turn, but only if none of that permanent’s loyalty abilities have been activated that turn. See rule 606, “Loyalty Abilities.”