This article introduces a known limitation about SCU (Source Class Usage ) on MX platform where DPCE and MPC are mixed; specifically speaking,  when  ingress interface is on MPC and egress interface is on DPCE.

Customer reports SCU does not work when ingress interface is on MPC and egress interface is on DPCE on MX platform.
This is a known limitation, addressed  in SCU function spec  and PR505057, but not in TechPubs, this KB is written as customer requested.

Problem is described as follows:
Related configuration to SCU is here:

jtac@MX480> show configuration groups dcu-scu interfaces 
ge-2/0/0 {
    unit 3101 {
        family inet {
            accounting {
                source-class-usage {
                    output;
                }
            }
        }
    }
}
xe-4/2/3 {                          
    unit 0 {                        
        family inet {               
            accounting {
                source-class-usage {
                    input;
                }
            }
        }
    }
}

Ingress interface xe-4/2/3 is on MPC and egress interface ge-2/0/0 is on DPC:

jtac@MX480> show chassis hardware | match fpc 
FPC 2            REV 13   750-021679   XB3934            DPCE 40x 1GE R
FPC 4            REV 38   750-028467   ABBT0269          MPC 3D 16x 10GE

SCU does not work here:

jtac@MX480> show interfaces statistics ge-2/0/0.3101    
  Logical interface ge-2/0/0.3101 (Index 326) (SNMP ifIndex 1383)
    Flags: SNMP-Traps 0x4000 VLAN-Tag [ 0x8100.3101 ]  Encapsulation: ENET2
    Statistics        Packets        pps         Bytes          bps
    Bundle:
        Input :       4008870        999    5924561731     11816496
        Output:       3984920       1005    5888850512     11894024
    Protocol inet, MTU: 3978
      Flags: Sendbcast-pkt-to-re, SCU-out
                                               Packets                Bytes
      Source class                 (packet-per-second)    (bits-per-second)

                        v4-scu-p2                    0                    0
                                  (                  0) (                 0)
                        v4-scu-p3                    0                    0
                                  (                  0) (                 0)
      Addresses, Flags: Is-Preferred Is-Primary
        Destination: 201.1.1.0/30, Local: 201.1.1.1, Broadcast: 201.1.1.3
    Protocol multiservice, MTU: Unlimited

This is a known limitation by design. SCU accounting occurs at egress on MPC, while at ingress for DPCE. When packets are sent from MPC to DPCE, each Destination Point Code assumes that the other one has incremented the counter.

Both ingress interface and egress interface for SCU are recommended to be configured on all MPCs or all DPCEs, other than a mixed scenario like MPC to DPCE (not  supported) or DPCE to MPC (inaccurate accounting).