EXPORTing procedures

Here's a puzzle. Does anyone have a good explanation? I have a source member containing two procedures - the main (entry) procedure and a subprocedure (PROC2). The main procedure calls the subprocedure.

If I code this as a Cycle Main program, all works well. Example:

**Free
Ctl-Opt DftActGrp(*No) ActGrp(*New);
// Main Entry procedure
Dcl-PR Proc2;
End-PR;
Dsply 'In main Proc' '*REQUESTER';
Proc2();
*InLR = *On;
Return;
Dcl-Proc Proc2;
// Internal subprocedure
  Dsply 'In Proc2' '*REQUESTER';
End-Proc;

If I code this as a Linear Main program, PROC2 requires the EXPORT keyword, e.g.

**Free
Ctl-Opt DftActGrp(*No) ActGrp(*New) Main(Proc1);
Dcl-Proc Proc1;
// Main Entry procedure
  Dcl-PR Proc2;
  End-PR;
  Dsply 'In main Proc' '*REQUESTER';
  Proc2();
  Return;
End-Proc;
Dcl-Proc Proc2 Export;
// Internal subprocedure
  Dsply 'In Proc2' '*REQUESTER';
End-Proc;

(The source will compile but the Bind step fails without the EXPORT keyword - "No definition was found for reference PROC2 in *MODULE".)
Why is the EXPORT keyword required?  (I've not been able to find any clear explanation in the manual - tell me what I'm missing.)

FOOTNOTE: If I modified the coding of the second example and EXPORTed PROC1, I could then create a *SRVPGM which did /not/ require PROC2 to be EXPORTED.  Odd!

Brian.