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applicationId
In build.gradle, applicationId is the fully qualified package name for your application that you specified during the New Project workflow.
Signing
All APKs (.apk files) must be signed with a certificate whose private key is held by their developer. This certificate identifies the author of the application. The certificate does not need to be signed by a certificate authority; it is perfectly allowable, and typical, for Android applications to use self-signed certificates. The purpose of certificates in Android is to distinguish application authors.