Thanks. I've massaged that paragraph. The new one reads:
OpenPGP combines symmetric-key encryption and public key encryption to
provide confidentiality. When made confidential, first the object is
encrypted using a symmetric encryption algorithm. Each symmetric key is
used only once. A new "session key" is generated as a random number for
each message. Since it is used only once, the session key is bound to the
message and transmitted with it. To protect the key, it is encrypted with
the receiver's public key. The sequence is as follows: