can someone clear up confusion about different user account types and subscriptions ?

hello, i'm installing a new server with Plesk 12.5.3 and am really confused how the user management works.

so i have several customers each of which has several domains with websites (no mail)

so i start by creating a customer, in the create dialog box i am forced to create a user for Access to Plesk
and then it asks me to create another user when i create a subscrition.

i already don't understand why i need to create two different users here (it wont let me select...

can someone clear up confusion about different user account types and subscriptions ?