The system can activate requirements (such as reservations or purchase requisitions) in different ways, depending on the settings in customizing and master data. Some of these ways are:
Immediately, by saving the project: This means that the system creates requirements for all materials assigned to WBS elements or activities as soon as you save the project. This option requires that you set the indicator "Requirements immediately" in customizing for your project profile.
When releasing a network or an activity: This means that the system creates requirements for all materials assigned to a network or an activity when you release them. This option requires that you set the indicator "Requirements on release" in customizing for your network type or activity type. Reference: https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/4dd8cb7b1c484b4b93af84d00f60fdb8/79ceb65334e6b54ce10000000a174cb4.html?&version=1909.002