what is a bound program access built in number

I am looking at the _LBCPYNV api.  it is used to copy one numeric variable
to another in a generic, variable descriptor kind of way.  The call looks
something like this:
   _lbcpynv( %addr(receiver): %addr(ReceiverScalarDescriptor):
             %addr(source): %addr(SourceScalarDescriptor)) ;

I notice on v5r1 that if the prototype does not have the correct signature
the compile of the module bombs the rpg compiler.

which gets me thinking that procs like _LBCPYNV are probably not called like
standard procs are called, that is instead of an full ile invocation stack
call, I would guess the "call" is actually just implemented as inline code.
Similar to how CEEDOD and other such procs are implemented.

I am curious to know if I am correct on that.

also, what is meant by a bound program access built in number?  The built in
number of LBCPYNV is 129. How is that number used?

thanks,

-Steve