Many thanks. Is that because it is used by more people than anybody else? What is it that determines that the Satoshi client "is" the protocol?
Of course the Satoshi client was the first and others such as Electrum or Multibit (at least AFAIA) are still relying on "full bitcoind" instances running on the servers that these "lighter" clients access (so they are effectively a protocol that sits above the bitcoin protocol).
What determines that it *is* the protocol was that is how the Bitcoin network came to be - the Satoshi code that people started using - as it was not released as an RFC or similar there simply was no other detailed documentation or software to use.