Different servers use different command syntax and support different sets of commands. The most common example is a server-side include command which simply expands to the contents of some given file and allows bits of HTML or text to be shared between pages for ease of updating. Other commands insert the value of an environment variable or the output of a shell command. These allow pages to be different each time they are served without requiring a CGI script.
Some servers distinguish SPML from HTML with a different filename extension, others use the execute bit of the file's permissions.