Which MS servers requiring users CAL's?

Physical or Virtual, still need the same CAL's
DC and Exchange with the correct CAL's (Windows User CALs and Exchange CALs), already covers file server access
MS SQL Express doesn't need CALs (as almost all Express software from MS is totally free)

Kimputer thanks,
Just to make sure I've understood, when the file server is a standalone server, and the DC and Exchange are different servers (to which I have CAL's), does the existing DC + Exch CAL's covers the file server usage?

BTW, the file server OS is 2012 R2, while the DC + Exch are 2008 R2 based.

Thanks

If the DC and File server are in the same domain, the xx User CAL's are for that domain. Therefore, even if you have 50 file servers, if the xx User CAL's covers the users, it covers the file server access.

Great news.
And if the OS are different like in our case (Exch are Windows 2008 R2, while the file server OS is 2012 R2), they're all using the same User CAL's?

Understood.
So, just to wrap it up:
If I currently have DC + EXCH user CAL's for Win 2008, and I intend to add a new 2012 server which will act solely as a file server, I still need to purchase enough NEW 2012 User CAL's for the amount of users which will use the new server. For example, if the organization has 20 users which are licensed to 2008 CAL's, but only 10 of them will access the new file server, I need to purchase 10 new User CAL's.

Did I got you right?

That's correct!

Thanks so much !