Number range question related to BCS - FM Area

Hi

We want to implement only BCS in my current project. BCS is only used for budget checking in cost element and cost center.

In the Maintain FM Area (OF15), there is a field 'No. range. no' that need to be inputted. I input 01 for example, and the error come out:

Number range interval 05 not defined for FM area 0100

Message no. FI653

1. How do I maintain that number range?

2. What is it use for? What if I didn't input?

3. What's the difference with maintain number range for input document?

4. What's the difference with maintain number range for family document?

Please explain in details

Thanks