Billing Document and Print Document

Hi,

I am new to the IS-Utility. But I have been assigned to do many RICEF. I was going through some functionalites of Utility. Please explain the difference between "Billing Document" and "Print Document". In SD invoice number and Billing number are same. But I am finding in Utillity the Billing document # and Print Doc. # are differenet.

In which scenario and why it is different.

Also, why the Billing Simulation is required.

I will be garteful to you if you kindly explain with business scenario. I have gone through this Forum and SAP help but I could not understand it.

Thanks,

Jans