What level of AES 256-bit encryption in Acrobat Pro XI

I thought I was being clear. Here is the TL/DR; version:

In Acrobat Pro XI, on the Protection accordion tab, the user has the ability to choose an Encryption Level.  First the user decides how:

Encrypt with Certificate

Encrypt with Password

We choose Password.

Then a dialog opens which allows the user to decide whether a password is required to open the document.

There is also the option to restriction Permissions with a  Permissions password.

Towards the bottom of the dialog, you can decide whether you want to encrypt all document contents, all contents except metadata,

or only file attachments.   In that bottom section of the dialog there is a Compatibility dropdown with these choices:

Acrobat 6.0 and later  Encryption Level: 128-bit AES

Acrobat 7.0 and later  Encryption Level: 128-bit AES

Acrobat X and later  Encryption Level: 256-bit AES

MY QUESTION: When we select Acrobat X and later (256-bit), which "flavor" or level of 256-bit AES does Adobe use to encrypt the PDF?

When we send the PDF to an affiliate, who tries to open it in their third-party (non-Adobe), supplying the password, they tell me that the encryption level is R6 when then are expecting R5, per the ISO standard. Those R5 and R6 designations do not mean much to me, since I am hardly an encryption expert,  but those are the terms that were mentioned to me.

Thanks