I sell and deploy a lot of Office 365 Business and Business Premium licenses, so I do know the basic differences, however I have a client that wants to collaborate with co-workers using Calendar, Task, and Contacts sharing, without any kind of 3rd party tool or service. But, they're contained and cannot get Business Premium, thus no email messaging under a domain they'd own.
So under Office 365 Business, since their under a single tenant still, can they share calendar and essentially collaborate using whatever tools are available? (Teams?).
Unlike Business Premium, Business does not contain Exchange (Email, Calendar, Contacts) SharePoint (Files) Teams (Collaboration Hub) Planner (Tasks) To Do (Tasks) or any collaboration apps except for OneDrive for Business.
Office 365 Business only contains the Office Apps (Word etc) and OneDrive (Personal File Storage). On the use cases above, business premium is 100% what they need - they can share calendar using Exchange and SharePoint and collaborate to a high degree with SharePoint and Teams.
Hope that answers your question
Best Response confirmed by
ViProCon (Occasional Contributor)