From RFC-3588 I found a statement which mention "The 'P' bit indicates the need for encryption for end-to-end security".
Where P - Protected Bit in AVP Flags.
Also section 4.5 Listed the AVPs which will be considered for Encryption.
Please refer two columns one is MAY which is under AVP Flag Rules which listed the AVPs with ''P'' Bit and last column of the table which is not part of the AVP Flag Rules which marked as "Encr".
I hope that is what you are looking for.
Note:- Same informations are not available in RFC-6733 which are mention in the same section of RFC-3588