Template:Parameter is used with in the template namespace to identify and categorize templates based on their usage of parameters.
Usage
It accepts one mandatory parameter, one of mandatory, optional, no, or none.
It also accepts one optional string parameter, any arbitrary text.
Return
None of these will work correctly in the documtation subpage, but will appear in the actual documentation It's invoked twice above, but for clarity sake...
{{Parameter|mandatory}} adds the template to the Templates With Mandatory Parameters category and returns:
{{Parameter|optional}} adds the template to the Templates With Optional Parameters category and returns:
{{Parameter|no}} or {{Parameter|none}} adds the template to the Category:Templates without Parameters category and returns: