APEX

Can someone clarify for me the difference between using the :APP_SESSION form for referencing substitution variables and the v('APP_SESSION') form? The reason I ask is that I had an application with a login form using the following call

wwv_flow_custom_auth_std.login(
P_UNAME => :P101_USERNAME,
P_PASSWORD => :P101_PASSWORD,
P_SESSION_ID => :APP_SESSION,
P_FLOW_PAGE => :APP_ID||':1'
);

This wasn't working (the session wasn't getting established properly). When I changed it to use v('APP_SESSION') for P_SESSION_ID, it started working.

I can't see any obvious reason why the 2 forms of reference would work differently - and I haven't been able to find a clear explanation of the distinction in the manuals (it's not easy to think of a good way of searching, so I may have missed something!)

Can anyone explain for me, please?

Thanks,
Paul.