Delegated Admin vs Global Admin

We provide you the flexibility of easily adding all of your customers via a Delegated Admin, or via a global admin in their office 365 tenant.

This describes what the difference is.

Delegated Admin: This is the preferred method. You simply use your single user account that has access to manage customers in the Partner Center and it allows you to add and bill all of your linked customers.

The Delegated Admin account uses the Microsoft Partner Center relationship to connect to your customers and get their licensing details to allow you to bill their licenses through to your PSA tool

We strongly recommend creating a user account specifically for this and granting it access to your partner console (no license is required in office 365)

Global Admin: This is an account in an Office 365 tenant. It only has access to that single tenant.
The account needs to have the Billing Admin and User managemetn permission as a minimum to be able to work with the licenses and user accounts.

You would typically use a Global Admin account, when you need to bill a customers Office 365 licenses and for some reason you are not allowed or able to have your 365 Partner account as a delegated admin on their Office 365 account.