What is the significance of the aliases field in the user preferences? Under:

System Preferences > Users &amp; Groups > Advanced Options > Aliases:

There are 2 lines. One is my Apple ID. The second is in the form

com.apple.idms.GUID


What is the significance of these fields?

I do not have my Apple ID set to login my computer.
A user name alias functions as a simple way to create a shorthand version of an account name. 

For example, if a users full account name is “Buscar웃SD” (which would be difficult to type every time to log in), one could setup an alias as “BSD” and just login with the shortened version instead.