Why doesn't Cristiano Ronaldo play in AC Milan in the Champions League?

According to the rules of the Champions League: a player can only represent one team in the Champions League in a season ~

In the first half of that season, he represented Real Madrid in the Champions League, so he couldn't play for AC Milan when he was in AC Milan ~ But Cristiano Ronaldo shared many goals for AC Milan in the league and made a very important contribution to AC Milan in the league.