I am coming up-to-speed on OpenSSL certificates. In all of my research I see where "data" is hashed to create a hash value that is used as the basis for a certificate's digital signature. This hash value is encrypted using the sender's private key, at which point it becomes the certificate's digital signature.
What "data" is hashed? Is it specific information from the certificate? Is it all certificate information? Does the the data include the certificate's public key? Or, is the data some random number or string that is generated by OpenSSL when the certificate is created? I'm assuming that the "data" is certificate data since one of the functions of PKI is to validate that the contents of a message (in this case a certificate) haven't changed since the message was created.