Transfer With Clearing VS. Transfer without Clearing

Hi,

In SAP There are the folloiwng two transactions

1. Transfer with clearing (F-30)

2. Transfer without Clearing (F-21)

Functionality standpoint, what is the difference between those two? When we should use which one?

Also, if we have an AR open item, can we create a new transfer document to transfer the amount to a new payer with reference to the existing document number belonging the open AR Item?

Thanks a lot,

Bijoy.