MoinMoin processing instructions have the same semantics as in XML: they control the paths taken when processing a page. Processing instructions are lines that start with a "#" character followed by a keyword and optionally some arguments; the keyword is always handled case-insensitive. Two consecutive hash marks at the start of a line are a comment that won't appear in the processed page.
Processing instructions this wiki knows of:
## any-text: comment
#FORMAT format-specifier: defines the input format for this page, known formats are:
#REDIRECT pagename: redirect to a different page (see PageRedirect)
#PRAGMA mode args: sets processing modes for this page; mode names are not case-sensitive, currently known modes are:
#pragma section-numbers off (or "0") switches off automatic numbering of section headers, and "on" or "1" enables numbering (the default mode is set by the show_section_numbers config option)
#DEPRECATED: Mark this page as deprecated, i.e. further changes to the page will create no backup versions and displaying it will show the current contents (usually the reason why this page is deprecated) plus the last backup. This effectively freezes the last version of the page, makes it unavailable for searches (no hits on stale content), but keeps that content available for viewing.
All PIs must appear at the very start of a page. An exception are comment PIs, those may appear anywhere within a page using "wiki" format (but not in pages using other formats).
For more information on the possible markup, see HelpOnEditing.