Manual:Hooks/ParserFetchTemplateData

Parameters

 * : 1-dimensional array of strings, representing template titles
 * : array map (contains key-value pairs) passed by reference, which can be modified by extensions to send their own template data

Use cases
Currently, TemplateData uses this extension. TemplateData will add each template title to the  map, and will assign one of the following values:
 * If a title is invalid title or is an invalid redirect title, gives a value
 * If a title doesn't exist, will assign it a key assigned with
 * If a title has invalid data from the database, or does not have a property in the  table, assign it as: