Final answer:
The Default profile is merged into each user profile when a user signs in to a Windows operating system. It serves as a template providing base settings for user accounts and is distinct from the Public profile, which contains shared items but is not merged upon sign-in.
Step-by-step explanation:
When a user signs in to a Windows operating system, the profile that is merged into each user profile is known as the Default profile. The Default profile acts as a template that contains the base settings for a new user account. Whenever a new user account is created, Windows uses this profile as a base to build the new user's profile, including settings such as the desktop layout, start menu configuration, and system permissions. It does not include personal data, which gets added as the user begins to personalize their account. The Public profile, on the other hand, contains items that are shared among all user accounts on the same machine, but it is not merged into individual user profiles upon sign-in.