Are union cards activated from your hand, or only from the field?

Question

My Kaiba deck has the cards Y-Dragon Head, Z-Metal Tank, and X something.

Z-Metal Tank, for example, says:

Once per turn, you can either: Target 1 "X-Head Cannon" or "Y-Dragon Head" you control; equip this card to that target.

It says other things, but this is enough to continue my question.

Can I activate this union effect when Z-Metal Tank is in my hand, as a sort of Special Summon, or can this only be done when it's already out on the field?

I guess this is a really basic question but neither the card text nor the beginner rule book accompanying the deck addresses this point.

Answer 1

Can I activate this union effect when Z-Metal Tank is in my hand, as a sort of special summon, or can this only be done when it's already out on the field?

This can only be done when it is on the field

The reason why is that it is because this is a Monster effect; when that card is in your hand you can't yet activate this effect, only when on the field is when you can do it.

This is also the standard way of work of all Union monsters. They can unite with each other or well come back as monsters, but those effects have to be activated on the field.

This we can see is backed up by the Union Monster ruling:

In order to attach a Union Monster to another monster, you must first have the Union Monster as a monster on the field. So you must play the monster normally, then you can attach it. You cannot play a Union Monster as an Equip Spell Card directly from your hand.