Can I split and merge subprojects?

Yes, both options are possible! Read here how you can split and merge.
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When you split a subproject from the main project, it turns into its own main project.

1. Open the list of subprojects.
2. Next to a subproject, click on the More actions button more_vert:
   
   
3. Choose call_split Split.

When you merge subprojects, all subprojects will be merged into one main project.

If you merge subprojects with different statuses, the merged project will get the status of the subproject you've selected before merging.

1. Open the list of subprojects.
2. Next to a subproject, click on the More actions button more_vert:
3. Choose merge_type Merge with others.