Measure session is a single endpoint that allows us to measure and attribute installs, opens and updates in a single call. Since data integrity is extremely important to us, using the measure session endpoint as an SDK-less solution requires commitments by you. As noted above, you'll need to contact your account manager for further details. The measure session endpoint should be notified on ALL app opens and app resumes. Each time we are notified of a session we will measure an open. Based on the data we have on the user, an install or update may be created as well.
For all users we measure the install which is the first app open we see for the user. Installs can be flagged as existing users and by default we don't attribute installs by existing users since they installed the app at an earlier point and we are just learning about them. If the user's app or device information changes between one measure session and another, the Measurement API automatically creates an update log.