How does targeting work in the Yu-Gi-Oh Trading Card Game?

Answer 1

When one or more specific cards a chosen by a card effect, it is considered "targeting". For example, when you activate "Soul Taker", you must target, or choose, the monster you want to use its effect on. When you activate "Sakaretsu Armor", you are targeting the monster who is attacking.

That means those card effects won't work if the targeted monster is gone. For example, if you activate "Soul Taker" on a Fusion Monster, and your opponent activates "De-Fusion", the targeted monster is gone so the effect would not activate. If your opponent activates "Book of Moon" to flip their monster face-down when you activated "Sakaretsu Armor", the card's target is technically not there.