What is the difference between a username and a Sage account / Sage ID?

I use Remote Data Access (RDA). Why do I need both a username and a Sage account / Sage ID? I always mix up my username and my Sage account / Sage ID passwords when I connect to the cloud

Username:

• A username is for opening a specific company file.
• It contains the user’s accounting rights for this company.
• The company’s built-in sysadmin user is the only user that can share or stop the sharing with Remote Data Access.
• A username password is made of 7 characters maximum.
• Having a username password is optional, except for the sysadmin user.
• You will be asked to enter a username every time you open a shared company.

Sage account / Sage ID:

• A Sage account / Sage ID is a single sign-on authentication scheme that allows you to access the cloud-connected services available with Sage 50.
• It is an email address.
• It is not specific to one company file.
• Every user that needs access to a shared file must have their own Sage ID.
• A Sage ID password is made of 8 characters minimum.
• Having a Sage ID password is mandatory.
• You will not be asked for a Sage ID and password every time you open the shared company.

Notes:

• it is possible for 2 users too use the same username to connect to the same shared company in multi-user mode, but with a different Sage account / Sage ID - this is not recommended
• If a user is connected in read-only mode, they won't appear in the list of connected users under File / View All Users
• 2 users cannot reconcile the same bank account at the same time in multi-user mode
• To avoid blocking other users in multi-user mode, each user should always connect to the shared company with their assigned combination of username + Sage account / Sage ID, as it appears in the users setup: