In previous versions of QGIS, when QGIS Desktop was closed the current profile would be saved in the profiles.ini configuration file, so the default profile was the profile used in the last instance of QGIS that was closed.
A new User selection policy feature allows users to select how default profiles are defined, in accordance with the following three options:

• Last Profile: The legacy mode which follows previous version behavior. This uses the last closed profile when opening a new instance of QGIS.
• Default Profile: Manually define a default profile among the existing profile that will always be used by default (without using a command line flag or electing a profile from within the QGIS UI)
• Let user choose at startup

When "Let user choose" is selected, if there are 2 or more profiles, a selection dialog is displayed before the SplashScreen which will allow the user to choose which profile to use or to create a new profile which will be loaded automatically.
Regardless of the selected policy, if only one profile exists it will be used, and if no profile exist, the default profile will be created and used.
User profile icons are also provided to allow users to visually differentiate between their active profiles.