This blog post informs about the transition regarding the possibility to bill services with a certain usage volume ("Usage-based billing").

In previous releases (until SAP S/4HANA Cloud 1905) the "Create Customer Projects" / "Plan Customer Projects" app in the context of "Professional Services" offered the possibility to bill services with a certain usage volume by using contract type Periodic Service in combination with the item usage "Usage-based billing".

As of SAP S/4HANA Cloud Release 1908 this combination was replaced by a explicit new contract type called "Usage-based billing".

This is described in the "What's New" article for SAP S/4HANA Cloud 1908:

And it is also part of a Feature Rollout Powerpoint document ("1908 Show & Tell Professional Services") which you can find in the SAP Cloud Community: (=> see slide 5).

Also the official documentation points out the usage of the new contract type, see: