Dear @IFS_Nuub 

1. in IFS, you can issue a PO with no part line ( starts from the PR ) , as long it is an adhoc purchase, one time.  You can use this function, to avoid the hassle of creating a part number, for one time adhoc purchases. Essentially you are purchasing it as a service ( of any type).  As long as you have identified the purchase group manually, you may process the PO and recieve the service ( in your case, an one time adhoc rental service). 
2. A rental purchase, and its associated process assumes you are purchasing this rental service, periodically and repetitively, and hence the need to define a proper part number, and predefine the associated purchase group with the part, thus streamlining the process. This is important to track the procure to pay process defined .
3. Do review the different types of rental purchases that you intend to do for your organization, and define atleast one part of each such purchase. 

Note: we recommend purchase parts even for services, with clear defined purchase parts ( IT services, legal, admin, consulting etc etc ) so that even the adhoc purchases are streamlined and remove the error associated with manually choosing the purchase group each time.

Regards