Info required on user creation

Hi,

I want to know the difference between the following two methods of user creation in ATLAS.

I navigated to the MongoDB Roles page via Clusters -> Security. Clicked on “Add new custom Role”.

Given a name to my new custom role. Then chosen the “Built-In Roles” -> readWrite from “Database Actions and Roles” for the “Action or Role” option and mention my Database “ABC”.

Then created a user and attached the custom role to the user. The user is displayed as readWrite@admin even though I have given access to only “ABC”

In the second method create a user and attach a default privileges with the same role and database used in the first method. Now the user is displayed as readWrite@ABC.

What is the difference between both the approach. Kindly explain the concept or let me know the docs that I have to refer.

Thanks.

Regards,
Subramanian S.