Historical Rules · 5 October 2018
Rule 702.43c
Official source text from the 5 October 2018 Rules release
Sunburst can also be used to set a variable number for another ability. If the keyword is used in this way, it doesn’t matter whether the ability is on a creature spell or on a noncreature spell. Example: The ability “Modular—Sunburst” means “This permanent enters the battlefield with a +1/+1 counter on it for each color of mana spent to cast it” and “When this permanent is put into a graveyard from the battlefield, you may put a +1/+1 counter on target artifact creature for each +1/+1 counter on this permanent.”