Hi @Imperial, welcome to the community and glad to hear you are having fun with Catchbook!
That's a great question. The rule is that you cannot delete the active page or binder. We did this for architectural and data protection reasons.
So it doesn't matter what is on the page, just if the page (or binder) is highlighted in blue indicating that it is currently active. If you switch to a different page or binder, you can then delete any items that are no longer active.
Hope that helps!