Historical Rules · 17 July 2015
Rule 306.5d
Official source text from the 17 July 2015 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 he or she controls any time he or she has priority and the stack is empty during a main phase of his or her turn, but only if none of that permanent’s loyalty abilities have been activated that turn. See rule 606, “Loyalty Abilities.”