This system has been determined superfluous and confusing. Please write /modifications/ and /plugins/ directory files instead.

In WSN Links versions 4.0.0+, WSN Gallery 3.0.0+, WSN KB 2.0.0+ and WSN Forum 2.0.0+ you can create XML files which instruct the script to apply a series of modifications. At Admin Panel -> Customizations -> Modifications, click the 'create new mod file' option.

There are several types of modifications you can request:
file replacement - Replace text with other text in a file. Use this to add new PHP code. The filename is relative to the base directory of WSN.
template change - Replace text with other text in a template. Use this to add new template code. The filename is relative to the template set.
template addition - Creates a new template with the specified content.
query - Run an SQL query. Since people use different tables prefixes, be sure to use {PREFIX} in place of wsngallery_ in the table name. Note that you can do more than one query at a time if you wish.
add language - Add a language item.

Mod files are useful for keeping track of changes you make so that you can easily re-apply them to another installation or a new version. They also make it easy for you to share your changes with other people if you wish to.