Question re Positive and Negative rules

A question regarding Positive and Negative rules...

What if no rules are set... does that imply that access is allowed for everything or does positive access have to be granted explcitly? If thats a yes... then why have positive rules at all?

I have read through the manual and did not find any reference to what happens when rules are omitted.

Woud very much appreciate the heads up on this.