[Marco van Herwaarden]

Best is not to use underscores within the prefix as the underscore is used as delimiter.

You can use underscores but you will need to keep in mind the following:
- If the first part (xxx_yyy_templatename) xxx is used in multiple prefixes (not recommended!!) templates will be placed in the first group that matches the prefix.
- Changing the order in which the prefixes are added to $only does make a difference.
$only[test] = "Main testgroup";
$only[test_sub] = "Sub testgroup";

will give different result as:
$only[test_sub] = "Sub testgroup";
$only[test] = "Main testgroup";