The value of AppId is stored inside uninstall log files (unins???.dat), and is checked by subsequent installations to determine whether it may append to a particular existing uninstall log. Setup will only append to an uninstall log if the AppId of the existing uninstall log is the same as the current installation's AppId. For a practical example, say you have two installations -- one entitled My Program and the other entitled My Program 1.1 Update. To get My Program 1.1 Update to append to My Program's uninstall log, you would have to set AppId to the same value in both installations.

AppId also determines the actual name of the Uninstall registry key, to which Inno Setup tacks on "_is1" at the end. (Therefore, if AppId is "MyProgram", the key will be named "MyProgram_is1".) Pre-1.3 versions of Inno Setup based the key name on the value of AppVerName.

AppId is a not used for display anywhere, so feel free to make it as cryptic as you desire. The value may include constants.

If you use a {code:..} constant to allow your user to customize AppId, you do not need to return the real value until just before the installation starts: if necessary you may return an empty or generic value at earlier times. If not empty, this value will only be used to attempt to restore previous install settings (like the settings stored by [Setup] section directive UsePreviousAppDir). If empty, it isn't used for anything.

The length of AppId with all constants evaluated should never exceed 127 characters.

AppId=MyProgram