From a practical point of view, if a piece of information can be found in the blockchain, it gives two kinds of a guarantee. On the one hand, the information exist, meaning that at verifying the whole chain, the information has to exist and has to be downloaded, otherwise the consistency can not be identified. On the other hand, the information is valid; actually the whole consistency of the blockchain gives the guarantee if the piece of information is valid. However there might be a solution to guarantee information consistency without the need of giving a guarantee for availability. If a piece of information is signed by the blockchain itself but it is not stored directly in the chain, it gives a guarantee, like an off-chain proof that the information was valid without storing that piece of information directly in the chain. Certainly, the situation will be a little bit more complicated if the blockchain can actually fork.