difference between "email" and "email1"

Hi everyone:

I'm just a Sugar starter and this may be a very silly question, please bear with me.

When I try to get Contacts Audit so that I can have the email changing history. I went to 'Advance Report' > 'create custom query' and I got below result after I run my query.

However, I noticed that the "field_name" has "email" and "email1" there. But I can only see "email"  for "Email Address" in "Studio > Contacts > Fields".

Besides, when I try to get "Email Address" via GET API endpoint, I need to use "email1" other than "email".

Can anyone please help to explain the difference between "email" and "email1"? And when should I use "email" and when should I go with "email1"?

Thank you very much!

Sophie