man 7 Xsecurity doesn't really say a whole lot more, just:
Code:
Server Interpreted
The Server Interpreted method provides two strings to the X
server for entry in the access control list. The first string
represents the type of entry, and the second string contains the
value of the entry. These strings are interpreted by the server
and different implementations and builds may support different
types of entries. The types supported in the sample implementa‐
tion are defined in the SERVER INTERPRETED ACCESS TYPES section
below. Entries of this type can be manipulated via xhost. For
example to add a Server Interpreted entry of type localuser with
a value of root, the command is xhost +si:localuser:root.
Not sure why it's on my 14.04 system not yours - the manpage is provided by package `xorg-docs-core` - perhaps that got pulled in because I have some X11 dev packages installed