^No prob! :>
It means Ancientirismon is a Red exclusive (its Digiscore can only be obtained in Red, so you can only perform its Digifusion there), and I think some of its components might be Red exclusive too?
So yeah, you can't get it in Blue unless you link or something.

It is a long time since I played though so I can't really verify anymore if all the info in the guide is correct, but hope it helps regardless!