Reading my colleague's blog I noticed links to useful pages on MSDN:
As you probably know, HRESULT values from MSMQ have the format 0xC00Exxxx.
In binary the first 4 bytes that is 1100 0000 0000 1110 which we can translate using the HRESULT link as follows:
Bit 0 - 1 means failure
Bit 1 - reserved (AKA I'm not sure)
Bit 2 - 0 means Microsoft-defined value (rather then customer-defined
Bit 3 - 0 means error code not an NTSTATUS result (so product-specific)
Bit 4 - not used if Bit 3 is zero
Bits 5-15 - 000 0000 1110 means the error code is for MSMQ only (FACILITY_MSMQ)
So that's my curiosity satisfied about where the numbers come from anyway.