This appears to be one of several Special (SPEC) instructions supported by the SY/MAX Model 400 processors...
LET is a Data Transfer instruction which transfers data from one register to another. BIN refers to the conversion described above. SPEC simply tells us this is a Special instruction.
In the example above, S10 is a Binary Coded Decimal value and is being converted to a Binary value and stored in S20.
"AB" is quite a generic reference to a large family of Allen Bradley controllers. But aside from all that, may I ask at least which controller architecture you are converting to (Logix 500 or 5000)? It may matter "if" you require further assistance with handling BCD conversion in the "AB" World.
Regards,
George