How do externals work?

[Belaid Moa]

Hello All,

 I am new to Revolution world. I get used to hypercards and XCMDs/XCFNs.
I am trying to move to Revolution but I found it difficult.
In hypercards I used to send data between stacks and XCMDs by using
hypercard libraries and Parameters.

I don't see how one can do this in Revolution? Do I need to create my own
library that allows such communication?

I found that both Alex and Trevor used XcmdGlue.c/.h and external.c/.h
to do such communication.

Alex&Trevor:

Am I right?

If yes, I really like to understand how does your solution work?

I apologize for taking your precious time. Your reply is appreciated and
thanks in advance.

With best regards.
Belaid Moa,
UVic