A host-based mechanism is a general purpose mechanism. It allows you to give access to a particular host, such that all users on that host can connect to the server. This is a weak form of access control; if a host has access to the server, all users on that host can connect to the server.
The Solaris environment provides the host-based mechanism for backward compatibility. Applications linked with a version of Xlib older than OpenWindows Version 2 or X11R4 do not recognize the new user-based access control mechanism. To enable these applications to connect to the server, a user must either switch to the host-based mechanism, or relink with the newer version of Xlib.