SUMMARY
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When you have 2 activities, it's almost impossible to discover the correct way of adding an application to a specific activity. Only by right clicking an opened application from the task, you can pin it to (or unpin it from) a activity.  
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STEPS TO REPRODUCE
1. Create 2 Activities.
2. Open the application launcher and right click on an application that you want to add to the Task Manager only for the current activity.
3. In the Context Menu, you have a single option to 'Pin to the Task Manager' (that does it to all activities).
4. To make things worse, the same context menu has 'Show in Favorites' that opens a sub-menu that allows you to select your desired activity.
5. After you pinned an application to the Task Manager, if you right click it, there is no simple option to unpin it only for a specific activity (the only option is to unpin it from all activities)
6. To correctly unpin an application from the Task Manager, first you have to open it, open the context menu for it (by right clicking the app on the Task Manager), select 'Pin to Task Manager' that will open a sub-menu where you can select only the activities you want.

EXPECTED RESULT
The entire workflow should be obvious and easy to use.

SOFTWARE/OS VERSIONS
KDE Plasma Version: 5.23.4
KDE Frameworks Version: 5.89.0
Qt Version: 5.15.2