Since the dawn of time (if the dawn of time was in 2011), assigning licenses in Office 365 has been a pain. It has never been complicated, but it has also never been a pleasant experience.
When you create a new mailbox in Exchange Online, that mailbox comes with specific settings, features, and protocols enabled. As an Office 365 administrator, you have the ability to go back and modify these settings later if—for instance—you don't want users to have their default mailbox size limit set at 100 GB, or if you want a specific retention policy applied to that mailbox.
In today’s cloud services world, organizations are always looking for ways to improve user experience while still retaining their corporate brand and image. Microsoft Office 365 has always offered some degree of banding for those who chose to implement Active Directory Federation Services (AD FS) for identity federation in their on-premises environment and as one would expect, it has matured over time to become easier and much more feature rich.
Fortunately, organizations who have chosen not to use identity federation also have the ability to apply custom branding to the Office 365 sign-in page where users are redirected when they are signing in to Office 365 services. It often becomes a little confusing because this functionality is actually configured in Microsoft Azure, not in Office 365 and is provided as part of the Basic or Premium edition of Azure Active Directory. As you probably know by now, Azure Active Directory is the identity and access management service used by Office 365 and when you sign up for an Office 365 tenant an Azure Active Directory free edition instance is automatically created for you.
If you would like to take advantage of the custom branding feature in Azure Active Directory, you need to upgrade to either the Basic or Premium edition of Azure Active Directory. You do of course have the ability to sign up for a 90 day trial as well.