Target 1 monster you control; reveal 1 Fusion Monster in your Extra Deck, and if you do, the targeted monster's name can be treated as the revealed Fusion Monster's, if used for a Fusion Summon this turn.
There's no real reason to use Fusion Tag for Dragon Master Knight. A Fusion substitute costs you one less card and doesn't force you to run BEUD. Granted, Fusion Tag would let you use "BEUD" + a Fusion sub, but BLS is more than searchable enough that it's better than Fusion Tag anyway.
you can subtitute for both xyz and vw, that means if you have 2 monsters on field and 2 fusion tags, you can summon xyzvw
Biggest question is, does this use the TCG or OCG Ruling in Duel Links? In the OCG, you can use it for Contact Fusions (such as XYZ) however in the TCG, you can. It’s due to how it’s worded. TCG version says “if used for a Fusion Summon this turn” whereas the OCG version says “if used as a Fusion Material this turn”, that doesn’t specify Fusion Summon, so it works with Contact Fusions (as they’re not actually Fusion Summons) only in the OCG.
Everyone commenting on this card doesn't seem to understand it's use, but a look at the monster images on the card should give you a big clue.
This card helps you summon E-hero fusion monsters that have requirements that are also fusion monsters. For example, "Elemental HERO Marine Neos" requires "Neo-Spacian Marine Dolphin" which is also a fusion monster you can only otherwise summon with Nex...but fusion tag allows any monster on your side of the field to pretend to be "Neo-Spacian Marine Dolphin"... at least for fusion's sake.