clarification on the argument info used for the deriveSecrets function

[hadi-ijreis]

the function takes 3 arguments, one of them is info
in the ChainKey class in ratchet, the argument given is "WhisperMessageKeys".getBytes
In another class, "WhisperRatchet".getBytes() was given as argument instead.
But I can't find the "WhisperMessageKeys" or "WhisperRatchet" values anywhere in the code, aren't they supposed to be constants defined somewhere?