The reference field is not something that is new in Joomla 1.6 and up. Joomla 1.5 had a 'Key Reference' field in the articles advanced parameters tab. What is different is where the field is now found and the way the data entered is utilised.
In Joomla 1.5 the content of the key reference field wasn't actually utilised at all. It was a spare field which you could use in a template override to quote an external reference, or change the name in the language files and use for whatever you want.
In Joomla 3 there is a new field in the Metadata Options called 'External Reference' which comes with the tooltip 'An optional reference used to link to external data sources'. The reason why the field has moved from parameters to metadata is obvious - it is now used to output a meta tag, but what is this xreference tag and more to the point, what is it used for?
Googling generally brings up nothing but a few Joomla related posts involving people asking what it is and what it is used for, or some Joomla related marketing nonsense by SEO's claiming to know what it is for or saying it's great whilst pussyfooting around it's actual usage.
So where do we start, given that the w3c validator (and the html5 version) show the tag as invalid? Well, the best place is probably the html specifications for meta tags but the results there are surprising in that there are only 5 officially recognised names.
Of course, just because something isn't official doesn't mean that it is not used or useful. Open graph tags are not recognised as official (and will not validate without the correct schema - if you actually care) but are clearly useful. Canonical links, alternative language tags etc etc etc the list goes on.
So where next? There is a list of recognised meta extensions by the whatwg (Web Hypertext Application Technology Working Group). This gets a bit more interesting as anyone can make a proposal, you can see what has been registered and also what has been turned down. The problem here is that xreference has neither been registerd nor refused, so where in blue blazes did this tag come from and why has it suddenly appeared in one of the worlds most popular content management systems?
My next (actually more like my first, but this article would have read a bit backwards) assumption was that it was something invented by Google that you could use to add rich snippets to search results. Whilst checking out tags that Google recognise turned up some interesting ones such as <meta name="google" content="notranslate" /> which stops Chrome and search results offerening to translate a page, xreference still didin't appear as an option.
Wikipedia - no mention; even on a keyword search, xreference aparently appears in not 1 article.
Bing? Surley not, and no, it seems not.
OSTraining's forum had this to say:
Honestly I can't believe there isn't much out there about external references.
But basically if you put an external reference to point to a URL you will use that meta data information that will override your main meta data for that specified article.
Surely not again, I find that very hard to believe. Firstly, the meta data displaying on my page is the data that I have entered in the relevant areas; Joomla has not magically scanned my external reference and replaced my pages meta information with that of the referenced page. Secondly, why would anything else looking at my page take another sites meta over that of my own, it's just not logical. Sounds like a fobbing off from someone who is supposed to be teaching people about Joomla but doesn't actually know the answer (made clear by the fact that they searched for information but couldn't find it).
So where are we now? Urg, nowhere it seems. It's like the whole thing was just made up by the joomla devs without any explanation.
If you have the answer, then please share us all in on it (and possibly Google and Bing too).