Wikipedia:HRT

The most common reasons a template or module is considered high-risk are:

There are no fixed criteria, and no fixed number of transclusions, that are used to decide whether a template or module is high-risk. Each page is considered separately. If a template or module relates to a biography of a living person that would strengthen any arguments in favor of its preemptive protection.

The risk of vandalism is increased for a template or module that is transcluded many times or is shown on high-visibility pages. The fact that numerous readers would see an edit to these pages provides an incentive to vandalize them and also magnifies the damage done by such an act. There have also been cases where well-meaning editors introduced an error to a template that broke millions of pages.

Although this kind of vandalism is reverted very quickly (often within one minute), it might be seen by thousands of viewers before it is removed; protection lowers this risk. An additional risk is that, before the vandalism is fixed, some of the pages may have their caches updated, and these may stick around for a long time after the vandalism is fixed. Because experience has shown that vandalism to templates is often performed from multiple autoconfirmed accounts, full protection or template protection may be required to prevent abusive editing.

Semi- and fully protected templates should normally have the {{}} template. It loads the unprotected /doc page, so that non-admins and IP-users can edit the documentation and categories. It also automatically adds {{}} to protected templates, which displays a small padlock in the top right corner and categorizes the template as a protected template. Only manually add {{}} to protected templates that don't use {{}} (mostly the stub templates).

The bottom of protected templates should usually look like this:

This process is not necessary for Lua modules, as the documentation from the module's /doc subpage is automatically shown on the main module page.

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