What is implicit withdraw and explicit withdraw?

What is implicit withdraw and explicit withdraw?

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:

  Route map for outgoing advertisements is out-cust-full
                                                  Sent            Rcvd
  Prefix activity:                          —-               —-
    Prefixes Current:             267441         10 (Consumes 416 bytes)
    Prefixes Total:             36446984         12
    Implicit Withdraw:     31214720          6
    Explicit Withdraw:       5210431          2
    Used as bestpath:             n/a          7
    Used as multipath:            n/a          0   

:

Implicit withdraw: Number of times that a prefix has been withdrawn and readvertised, this number is smaller than Prefix Total(sent in this case). 

Explicit withdraw: Number of times that prefix is withdrawn because it is no longer feasible.