title: How does Firebase Analytics define a "user" (for purposes of Audience)?
body: <p>I've been told that, "Once a user is in an audience, they will forever be in that audience". I'm curious how long is "forever," in the context of a user? For instance, if a user changes phones, or uninstalls the app and then re-installs later, does the user persist across those transitions, or is each of those a separate user? And if I have multiple apps in one Firebase project, does the user appear in the same audience in all apps within that project?</p>
  
  <p>What does Firebase Analytics consider to be a unique/new user?</p>
  
  <p>One use-case I'm looking to try with Audience, is I want an audience of "existing users" (defined by "first_open_time &lt;= {some date}") and "newer users." I have some changes I want to implement for new users, but I want to let existing users have "legacy" behavior for a while, transitioning them more slowly. If a person was in the "existing user" audience, then got a new phone, or uninstalled and later re-installed the app, would they suddenly be a new user, and therefore no longer be in the "existing user" audience?</p>
  
  <p>Another use-case: I have an existing app, and I am writing a whole new app, which will have a new package name, and will therefore be a new install for all my existing users. I want to transition them all to the new app for free, and the best way I can think to do it is have both the old and new app be in the same Firebase project, and put users of the old app in an audience, and when they run the new one, if they are in the "old app users" audience, it will give them the premium features automatically.</p>
  
  <p>To be confident in either use-case, I need to better understand the definition of "a user" for Firebase - does it persist across uninstalls, transition between devices, and can you share audiences between apps in a project?</p>
  
  <p>Thanks!</p>