Create Dunning Plan -> Where is the '*Min Days" field used for?

Hi experts,

Till now I was convinced that the setup related to days overdue have to be setup as *Min Days. 

 

So for example Dunning Level 1 -> After 21 days over due

 

This does not seems to be the case.  I need to know where I can define between which days each dunning level is used when setting up staged dunning.

Where do I define the days per dunning level?

 

For example Dunning Level 1 -> After 21 days overdue

                     Dunning Level 2 -> After 60 days overdue

                     Dunning Level 3 -> After 80 days overdue

                     Dunning Level 4 -> After 100 days overdue

 

…and what is the field *Min days is used for?