Itemid is a particularity of Joomla/Mambo... that some people find useful and others not...
So I'll try to give a neutral answer (my own understanding and opinion only):
It's in fact the menu id.
In my understanding, it serves three purposes:
1) to make Joomla display the correct menu item and link corresponding to what has been clicked.
- This is useful if you have more than 1 menu item corresponding to a content item.
- But if you click a content item navigation, it has the reverse effect to NOT highlighting the corresponding menu item (if you have one)
2) to help for the implementation of the "Back" button (not sure of that one)
3) to check for the permissions of that access to that content item from that menu/link using the menu-item access permission setting
- This is useful if all menu items & all links have correct settings
- It is a major source of errors in admin configurations
- It is a week access permissions control, that the better 3PD components double with their own access permissions checkings.
EDIT: And a fourth (fergotten completey! even if I knew it !): adding also here to complete the list for further reference:
alwarren wrote:
...
4) It is the foundation for displaying modules. Itemid determins which module is displayed on a page. When you select a menu item from the module edit screen, it adds the menu item id(s) and the module id to the #__modules_menu table. That table is checked each time a page is displayed to determine whether a module should be displayed or not based on Itemid and the modules id.
Itemid has also an side effect that you can see better when activating SEF and even better when using a joomla-URL compliant SEF extension like 404sef: each page may have more than 1 url to it, if there is more than in total one of either menu item or link to it. Some people believe that if there are more than a few pages with different URLs but with same content, it might decrease your ranking on search engines. With up to 3-4 different links, I haven't seen this (one of my such sites has rank 3-30 of 30'000'000 pages, but I didn't try with 30 different itemIds.
)
Dear Core-team:
- please correct me/complete the list above if incomplete or inaccurate
- For the main purpose ( 3) ) it would be highly desirable to solve that problem (and increase site security) for the content itself without looking at the menus permissions. Maybe part of ACLs step 2 ?
