If I compare them in the Key metrics report, the numbers are totally different: On one day I have 3 times more Daily unique visitors than Daily engaged users. Or I have almost double 1st Visits than 1st Launches... How is that possible? Shouldn't the numbers be exactly the same?
I think the underlined explanation would clear some things about visits and launches:
There are two factors in the difference between a visit and a launch. These differences relate to the timeout value that ends a visit or a session. A new launch in an app is synonymous with a new session from the SDK. The first factor is how visits are calculated. Visits are calculated server-side by the Analytics platform based on when the platform sees data coming in. The first server call starts the session and lasts until the last server call without exceeding the visit timeout. The typical timeout is 30 minutes in a report suite. Visits is a web-centric metric but it is still useful in apps to compare app bouncing. App bouncing is how often users are in and out of your app. Visits will tell you the total duration of the app while the session length, described below, will tell you the time the app is actually in focus. The second factor is the session length. The SDK sends in a metric called Launches. The logic to trigger a new launch is if the user has been out of the app for longer than the timeout value. The default timeout value in the SDK is 5 minutes (300 seconds in the configuration file). This can be changed and might be different for each app. Example 1: A user is in the app for two minutes, backgrounds the app for two minutes, returns to the app for two minutes, and then leaves the app for the day. There is one visit and one launch. The visit length is 6 minutes (the time between the first server call to Adobe and the last). The session is a four-minute session because Adobe subtracts the two minutes the app was in the background. There is one launch (or session) metric because the user came back into the app before the SDK timeout value was exceeded. Example 2: A user is in the app for two minutes, backgrounds the app for tem minutes, returns to the app for two minutes, and then leaves the app for the day. There is one visit and two launches. The visit length will be 14 minutes. There are two launches/sessions, each for two minutes There are two launches because the user exceeded the five-minute timeout while the app was in the background.