A question about span

I am a newbie to telephony/IVR, to be honest.
And I can't find anyone around who knows telephony/IVR well.

So, could someone explain to me the following question, or at least tell me
where i can found the answers?
I have did some search on internet, especially wikipedia. Unfortunately, I
can't find a quick answer.

My first question is whether span is a property of IVR card, like those
conform to capi20?
Is the number of ports in a span fixed? Or we could change it as we want? I
guess it's predetermined.
I've seen a function call:  capi20_get_profile(span,
(CAPI_MESSAGE)&spanprofile);
and spanprofile has a member called nbchannel.
However in the driver of dialogic, the number of ports seems to be hard
coded.

Could every device have span? I guest not.
Coz I found that not every driver's startDriver() function called
BayonneSpan's constructor.

Thanks in advance!

-- 
Zhang Le, Robert
Linux Engineer/Trainer