W-Catapulta Ala

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.
You can Summon a Union Monster and attach it to another monster right away. You cannot attach a Union Monster and detach the same Union Monster during the same turn, unless a card effect specifically allows it (such as “Combination Attack”).
Union Monsters only grant their special effects to a monster when equipped to that monster using the Union Monster’s effect. So if “Relinquished” equips itself with a Union Monster, it gains none of the Union Monster’s bonuses. Also, if “Union Rider” equips itself with a Union Monster, “Union Rider” gains none of the bonuses. The Trap Card “Formation Union” is considered to equip the Union Monster ‘by its own effect’ so the bonuses do activate.
A Union Monster that is attached to a monster is treated as an Equip Spell Card and can be destroyed with “Heavy Storm”, etc. You can use “Magic Reflector” to place a counter on an equipped Union Monster and, if the equipped monster would be destroyed in battle, you can remove the counter instead so neither the equipped monster nor the Union Monster are destroyed.
A monster can only be equipped with 1 Union Monster at a time, but can still be equipped with other Equip Spell Cards such as “Axe of Despair”, etc.
If an equipped monster is taken by the opponent (such as with “Snatch Steal”), the Union Monster remains where it is, although it is still equipped to the monster, and the player who controls the Union Monster can detach it and it is Special Summoned to that player’s side of the field. (So if I have “X-Head Cannon” equipped with “Z-Metal Tank”, and my opponent uses “Snatch Steal” on “X-Head Cannon”, on my next Main Phase I can detach “Z-Metal Tank” and it is Special Summoned onto my side of the field because I still control it).
If a Union Monster turns into an Equip Spell Card, or reverts back into a monster, effects are reset. For example, if I use “Call of the Haunted” to Special Summon a Union Monster, then attach it to another monster, it is no longer affected by “Call of the Haunted” (which remains on the field meaninglessly and, if destroyed, does not destroy the Union Monster). If I use “Limiter Removal” on a Union Monster, and equip it, it is not destroyed at the end of the turn, and if I turn it back into a monster its ATK is no longer doubled.