This topic has been covered before (here and here), but I was recently in a conversation where this was brought up, so thought I’d do a short refresh.
To sum it up quickly; you need to purchase server licenses for your products, but depending on the Office 365 licenses you have, they cover on-premises usage rights for your employees, so no need to purchase duplicate CAL’s.
It’s all listed in the Product Terms document which you can download from https://www.microsoft.com/en-us/licensing/product-licensing/products.aspx. The document covers use rights for other products as well and is the go-to guide on licensing.
In the latest version dated June 1st, 2017 a fair bit down in the SharePoint section you see that SharePoint Online Plan 1 cover standard licenses and Plan 2 cover enterprise features of SharePoint 2016 on-premises. This means, you still need to purchase the appropriate server licenses, but if you have Office 365 E1-E5 licenses, you are covered on the user licenses – as E1-E5 cover SharePoint Online Plans. E1 only covers base or standard SharePoint features (see table at the end).
You can find the same information in the document for licenses regarding Exchange and Skype for business. There’s also a note pointing to Appendix A which puts it all in a nice table, and I’ve highlighted the ones for SharePoint use right as an example. The top row lists all the different licenses you can have for users, and the blue squares show where they apply as valid on-premises licenses.
The Base entry is standard functionality, and the additive is for enterprise functions.
This means that if you have an Office 365 E1 license, you can use SharePoint Standard functionality, while E3, E4 and E5 give you usage rights to Enterprise features as well like enterprise search, e-discoery, InfoPath services etc, all listed in 3.2.1 in the first image.
