Creating user without mentioning the login name

As a sysadmin, I create a login:

CREATE LOGIN BobSmith WITH PASSWORD = 'mypassword'

I now am in the Sales database and execute this:

CREATE USER BobSmith

What happens?

The user BobSmith is created and mapped to the login BobSmith

If a login name exists with the same name as a user, it is automatically mapped.

Source: SQL Server Central QoD(2015/12/29).

Ref: CREATE USER - https://msdn.microsoft.com/en-us/library/ms173463.aspx