BCD[_] changes its argument to BCD representation, interprets it in binary and outputs that value.
Let us say #149 contains 10 (in BCD[#149]).
BCD representation of 10 is 0001 0000.
The binary interpretation of 00010000 is 16.
So, 16 is output. Thus, BCD[10] = 16.
BCD and BIN functions are used in association with system variables for input/output interface signals.
For more information, read the attachment.
Sinha