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: https://help.sap.com/doc/bf7675d17e574ea98efe979dfc02256d/1/en-US/2084326245d841fb9e4d5fcdffc7797d.html
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:
https://s4hanacloud.community.sap/groups/HzKQzw50FoTkrVY08r6iC1/documents/Op89C7wZhHFparkB6GVll0/slide_viewer (=> see slide 5).
Also the official documentation points out the usage of the new contract type, see: https://help.sap.com/viewer/29cf986299714386847f4d4c9bb86994/1908.500/en-US/3f6d30bca32240d69bb9cf008aa5e23a.html