What is the exact meaning of revocationWarning and revocationNotification in RFC 3161? The RFC 3161 (Internet X.509 Public Key Infrastructure Time-Stamp Protocol) specifies that a time-stamping response has a PKIStatusInfo and, optionally, a timeStampToken. In turn, the PKIStatusInfo contains a PKIStatus, which is an integer in [0, 5]. For the values 4 and 5, the following descriptions are provided:


  revocationWarning      (4),
  -- this message contains a warning that a revocation is imminent
  
  revocationNotification (5)
  -- notification that a revocation has occurred


Both descriptions refer to "a revocation". What is this revocation referring to?

My guess is that it refers to the revocation of the TSA certificate... Is this correct? May any other revocation cause this response?
According to a discussion on the PKIX mailing list, it means the TSU (Time Stamping unit) certificate which signs the time stamp responses.