Once extra cards being looked at have touched a new private or secret group from their original source, they become part of that group. If the extra cards have not touched a new group, this is instead a Looking at Secret Information error. See 1. For example, if a player plays a Stacked Deck and adds a fourth card to the private set of cards they are looking at, it is Looking at Extra Cards. If they correctly look at the top three, but knock the top card of their Main Deck over without adding it to the private group, it is Looking at Secret Information.
§ 702.5.b Tournament Rules PDF (officieel)
§ 702 Game Play Errors
§ 702.5 Looking at Extra Cards [Warning]: While taking a game action that allows a player to look at cards from a deck, they look at more cards than they are entitled to.
