DefaultAccount
The DefaultAccount, also known as the Default System Managed Account (DSMA), is a built-in account introduced in Windows 10 version 1607 and Windows Server 2016. The DSMA is a well-known user account type. It is a user neutral account that can be used to run processes that are either multi-user aware or user-agnostic. The DSMA is disabled by default on the desktop SKUs (full windows SKUs) and WS 2016 with the Desktop.
The DSMA has a well-known RID of 503. The security identifier (SID) of the DSMA will thus have a well-known SID in the following format: S-1-5-21-<ComputerIdentifier>-503
The DSMA is a member of the well-known group System Managed Accounts Group, which has a well-known SID of S-1-5-32-581.
The DSMA alias can be granted access to resources during offline staging even before the account itself has been created. The account and the group are created during first boot of the machine within the Security Accounts Manager (SAM).
How Windows uses the DefaultAccount
From a permission perspective, the DefaultAccount is a standard user account. The DefaultAccount is needed to run multi-user-manifested-apps (MUMA apps). MUMA apps run all the time and react to users signing in and signing out of the devices. Unlike Windows Desktop where apps run in context of the user and get terminated when the user signs off, MUMA apps run by using the DSMA.