Last modified: 2010-03-25 16:52:15 UTC
{{INCLUDING_PAGE}} for the name of the including page of a template when that page is included by another page (it is not {{PAGENAME}}). Motivation: A page (P) may include sub-pages (P1 ... Pn) and the sub-pages could have their own talk pages. A header template (H) included by P1 ... Pn would need to refer to [[{{TALKSPACEE}}:{{INCLUDING_PAGE}}|talk]] so a reader of P could immediately reach the individual talk pages conveniently.
{{INCLUDEDPAGE}} would seem like a better name, although probably someone can do better than that ({{REALLYPAGENAME}}? :P).
{{INCLUDEDPAGE}} takes the view of the author of page P. That doesn't make sense when INCLUDEDPAGE could be P1 ... Pn and these could again have sub-pages P1.1 ... P1.i ... Pn.1 ... Pn.j. What would be the value of INCLUDEDPAGE? For the template it would make sense to have an {{INCLUDING_PAGE}} to allow it to distinguish based on what page it is included by. If the page generation doesn't work that way (has no way to change the value of {{INCLUDING_PAGE}} for different templates included in the same page P) then it would have to be a function {{#function-including-page:<level>}} instead (e.g. with level as an identifier for the number of inclusion levels the including page is removed from the template).
{{INCLUDEDPAGE}} is just creating {{subst:PAGENAME}} variable that updates when the template is moved. {{INCLUDINGPAGE}} is not possible with the current preprocessor and would require a lot of work to facilitate.