When setting up new projects in Core, say residential projects, the existing standard project templates available might not fit your specific needs. Sometimes, you might start off billing the clients in the early phases on an hourly basis and then switch to some other contract type or billing arrangement.
In such cases, you should create your own customized project template to accommodate such changes in billing within the same project. In Core, you can handle such situations by creating the required project structure and then saving that as a new template.
When done, go into the General > Details screen of the main/parent project and click Actions > Save as Template. You can use this new template for creating your other similar projects.
Check out the Projects help for more.