What is Target App ID

The property target app id is new app attribute added in June 2018.  The definition is documented in Administer Qlik Sense Enterprise > Qlik Sense Enterprise on Windows >  Managing a Qlik Sense enterprise on Windows site > QMC resource overview > Apps

Target App ID:

The App ID of the published original app that you have duplicated. When you create copy of a published app, and want to replace the original app, the target app ID is used to identify it.

Notice, This is not something the user or client uses to decide anything - it's automatically used in the backend to select the target app to replace.

The intended workflow is:

1. Publish an app (app 1)
2. Duplicate it (creating a working copy, app 2, which will have target app id pointing to app 1)
3. Continue development on app 2
4. Publish app 2, which will automatically replace app 1

If delete app 1, target app id on app 2 will be cleared - since the target app doesn't exist anymore. 

If export app 2, and import it again, its target app id will also be cleared, and in this scenario using the QMC publish/replace can manually select to replace app 1.