To be properly Special Summoned, Unification Monsters must first be Unification Summoned. If they leave the Extra Deck without being Unification Summoned, they cannot be Special Summoned while they are banished or in the Graveyard.
Like other Extra Deck monsters, Unification Monsters cannot exist in the hand or Main Deck (even with card effects). If a Unification Monsters would be moved to the hand or Main Deck, it is returned to the Extra Deck instead. If a card like " " or " " is applying, this will override the Unification Monster's ability to return to the Extra Deck.
A Unification Summon is the act of Summoning an Unification Monsters from the Extra Deck by using the specified monsters as Unification Materials. Usually the materials are sent to the Graveyeard, but in some cases (specified in the card's text) the materials can be used differently (Ex. An Unification (Xyz) monster normally have it's Unification Materials atached to it as Xyz Materials.
Alternative Unification Summon
If there is an face-up "Inifinite Unification" in the field, both players can perform a Unification Summon using their alternate invocation condition. In this case, follow the instructions in the card text
ATK and DEF
The ATK and DEF of a Unification Monster equals the average of the original status of its Unification Materials +1000. Unification Monsters can break the 5000 ATK rule.
The card frame of a Unification Monster is a fusion of whatever type of Monster Card their Unification Materias are. (Ex. A Unification Monsters that uses a Fusion and a Xyz Monster as Unification Materials has it's card frame half violet, half black). The only feature that does not change is the edge of the card itself, which is white rather than black
Unification Monsters are treated as being of the same type of Monster Card as its Unification Materials, for all purposes. This types are listed between parenthesis in the Type/Ability line. (Ex. Dragon/Unification (Fusion, Xyz)/Effect). Because of this, Unification (Normal) are always treated as non-Effect monsters, Unification (Dark Synchro) has Negative Levels, Unification (Xyz) can have Xyz Materials, etc.
The effects of Unification monsters are improved versions of the effects of its materials
The Level/Rank of a Unification Monster equals the average of the original level of its Unification Materials rounded up.