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Setting the policy fixes which application identifiers Google ChromeOS shows as pinned apps in the launcher bar, and users can't change them.
Specify Chrome apps by their ID, such as pjkljhegncpnkpknbcohdijeoejaedia;
Android apps by their package name, such as com.google.android.gm;
web apps by the URL used in WebAppInstallForceList, such as https://google.com/maps;
System Web Apps by their snake case name, such as camera.
Isolated Web Apps by their web bundle ID, such as egoxo6biqdjrk62rman4vvr5cbq2ozsyydig7jmdxcmohdob2ecaaaic.
Leaving it unset lets users change the list of pinned apps in the launcher.