Historical Rules · 15 July 2011
Rule 706.11
Official source text from the 15 July 2011 Rules release
An effect that instructs a player to "cast a copy" of an object follows the rules for casting spells, except that the copy is cast while another spell or ability is resolving. Casting a copy of an object follows steps 601.2ag of rule 601, "Casting Spells," and then the copy becomes cast. The cast copy is a spell on the stack, and just like any other spell it can resolve or be countered.