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As far as I know, if I have some tags in a question, system will choose the tag which has more questions as main tag (shows in title of page)
But, sometimes this is not correct. For example, I have a question about PhoneGap which applies to both android and iOS. So my question is about PhoneGap, not Android. but system chooses Android as main tag...
Is there any way to manually choose the main tag of a question? if not, Isn't it better to have a feature like this?
What you are referring to is only the text appended to the title of that specific page. The so called "main" tag of a question doesn't really exist.
You'd be able to find a question tagged with android, ios or phonegap just as easily within each of those tags and also with any combination of the tags. I do not believe that the choice of what tag appears in the title of the page has any effect on the post, the question or the answers.
As mentioned in a comment by Bart, this is search engine optimization wizardry and is meant to help search engines to see what information a certain page contains in order to provide more accurate search results. A post was made on Webmasters.SE about this specific feature on the Stack Exchange network and from the votes it looks like it is some accurate information...
