This is a meta-template which helps other templates detect whether they are used in a front-facing namespace - main, Draft, Wikipedia, Portal, Help, and MOS. Additionally, it also includes the Template namespace.
This is useful for templates like infoboxes where the need to detect incorrect usage of parameters should cover more than just the main space.
Do not use this template to wrap elements that output maintenance categories, since that can place template /testcases and template documentation pages from Template space in those categories. Testcases pages often contain demonstrations of deliberate errors that should not be "fixed", and documentation pages often deliberately contain demonstration values or raw parameter names in values, so listing them on cleanup category pages is undesirable.
Usage
{{Front-facing other|action if on a front-facing page|action if on another type of page}}