Summoning condition

Summoning Conditions are required prerequirites for Summoning a monster to the field.

For example, in the case of Chaos Sorcerer, it's inability to be Normal Summoned or Set, and it's requirement of removing 1 Light and 1 Dark monster in your Graveyard from play to Special Summon it, are the card's Summoning Conditions.

A card that includes 'This card cannot be Special Summoned except...' in it's card effect text, means that the monster can only be Special Summoned via the card effect listed. Even if it is sent to the Graveyard after having been properly Special Summoned via that effect, it cannot be Special Summoned from the Graveyard, using cards like Monster Reborn or Premature Burial.

The only way around that is to use Level Modulation, which has the special effect text, "Ignoring the summoning conditions". However, this will only work if the monster was properly Special Summoned via it's relevant card effect first. If that card was discarded to the Graveyard via a card effect, of a card cost (Cases in point: Spirit Reaper or Lightning Vortex), then it cannot be Special Summoned with Level Modulation, because it was not properly Special Summoned first. Level up! will not work, period, because card's in the hand or Deck have their Summoning Requirement's 'reset', if the card in question was properly Special Summoned first, and then sent back to the Deck via a card effect, like Fiber Jar.

If a card has 'This card can only be Special Summoned by...' in it's card effect text, that means that, once it's fulfilled it's specific summoning requirements, and later sent to the Graveyard and/or Removed from Play, it can be Special Summoned as many times as you have reserve card effects to activate them.