A Signature corresponding to a Key. It is a sequence of bytes holding a public key signature from one of the three supported systems (ed25519, RSA-3072, ECDSA with p384). Or, it can be a list of signatures corresponding to a single threshold key. Or, it can be the ID of a smart contract instance, which is authorized to act as if it had a key. If an account has an ed25519 key associated with it, then the corresponding private key must sign any transaction to transfer cryptocurrency out of it. If it has a smart contract ID associated with it, then that smart contract is allowed to transfer cryptocurrency out of it. The smart contract doesn't actually have a key, and doesn't actually sign a transaction. But it's as if a virtual transaction were created, and the smart contract signed it with a private key. A key can also be a "threshold key", which means a list of M keys, any N of which must sign in order for the threshold signature to be considered valid. The keys within a threshold signature may themselves be threshold signatures, to allow complex signature requirements (this nesting is not supported in the currently, but will be supported in a future version of API). If a Signature message is missing the "signature" field, then this is considered to be a null signature. That is useful in cases such as threshold signatures, where some of the signatures can be null.
Smart contract virtual signature (always length zero)
Ed25519 signature bytes
RSA-3072 signature bytes
ECDSA p-384 signature bytes
A list of signatures for a single N-of-M threshold Key. This must be a list of exactly M signatures, at least N of which are non-null.
A list of M signatures, each corresponding to a Key in a KeyList of the same length.