Unpublishing an activity removes the particular activity from the XML file. This is a Pro feature and is only available to the admin user of the organization.
You can unpublish only published activity by clicking the unpublish button.
There can be two scenarios.
1. Main activity file is already published to Registry
The registry already has the link to the main activity file, unpublish will remove the activity content from the main file, which will also be reflected in the IATI activity file.
2. Activity is published but is not published to Registry
Unpublish will remove the individual activity from the main activity file, so when the user publishes the file to registry, the changes will be reflected on the main activity file.