You have to consider that you can use an indicator to determine that only coded project numbers may be used within the project system. A coded project number is a project number that follows a predefined structure defined by a project coding mask. A project coding mask consists of a project coding key and a coding mask that specify the length and format of the project number and the WBS element numbers. You can use an indicator in customizing to restrict the creation of project numbers that do not follow a project coding mask. Reference:https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/4dd8cb7b1c484b4b93af84d00f60fdb8/df72bb53707db44ce10000000a174cb4.html