http://mcpmag.com/articles/2011/02/...-available.aspx

Server Differences
The three servers can all be used by small organizations, but the choice depends on the number of PCs or users and what network functions are required. In terms of organizational size, Windows Home Server supports up to 10 PCs. Windows SBS 2011 Essentials supports 25 users max, while Windows SBS 2011 Standard provides access for up to 75 users or devices.

Licensing also varies among the three servers. Those wanting to expand the use of Windows Home Server to more than 10 PCs have to buy another server, according to a Microsoft forum response dated April 8, 2010. Windows SBS 2011 Essentials does not bear Client Access Licenses (CALs) costs, but Windows SBS 2011 Standard does have those costs, plus a server license cost. According to Microsoft's explanation of SBS licensing, buyers of Windows SBS 2011 Standard get a license for one server and five CALs. Organizations can choose the CAL types used with Windows SBS 2011 Standard, either based on the number of users or on the number of devices.

Both Essentials and Standard users have the ability to add a server license and CALs through an offering called "Windows SBS 2011 Premium Add-on," which provides access to SQL Server 2008 R2 for Small Business. Leworthy said that Microsoft devised this Add-on to address the costs that small businesses faced with adding an extra server.