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Well Cisco says (in http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoSIntro.html):
Note 
The QoS Baseline recommends marking Call-Signaling to CS3. However, currently most Cisco IP Telephony products mark Call-Signaling to AF31. A marking migration from AF31 to CS3 is under way within Cisco, but in the interim it is recommended that both AF31 and CS3 be reserved for Call-Signaling and that Locally-Defined Mission-Critical Data applications be marked to a temporary placeholder non-standard DSCP, such as 25. Upon completion of the migration, the QoS Baseline marking recommendations of CS3 for Call-Signaling and AF31 for Locally-Defined Mission-Critical Data applications should be used. These marking recommendations are more in line with RFC 2474 and RFC 2597.
So, AF31 will be used for Locally-Defined Mission-Critical Data. NB: I have not looked at the two RFCs, but I would guess AF32 and AF33 would be used for different drop priorities for such traffic. Signaling you want to avoid dropping at all.
