indicates a "that the destination of that hyperlink is a license for the current page." whereas rel-ecolabel would indicate that the destination of that hyperlink is an ecolabel for a product, company or service listed on the page.
In this case I am uncertain if rel-license defines an authoritative source for license information. The difference with rel-ecolabel is that we would define an authoritative source at www.ecolabelling.org. We'd do this because there are many ecolabels 300+ and very few display their information in a standardised way. That being said, we're not beholden to this approach.
This would need to have the option / ability to point to the issuing labels site as well. Or if www.ecolabelling.org want to become the source, it should have a simple an quick method of adding labels into the system that aren't listed .. eg. a lot of UK based certified labels are currently not on the site. Would there be a conflict if I wanted to point the URL at the label associations website instead of www.ecolabelling.org (asked for discussion purposes)
Label example: Vegan Society Trademark: http://www.vegansociety.com/business/trademark/
ecolabel entry @ ecolabelling.org eg: http://ecolabelling.org/ecolabel/energy-star/
ecolabel entry @ greenerchoices.org eg: http://greenerchoices.org/eco-labels/label.cfm?LabelID=15&searchType=Label%20index&searchValue=&refpage=labelIndex&refqstr=
ecolabel ISO types eg: http://www.gen.gr.jp/eco.html
The Dublin Core conformsTo term appears to already solve a more general problem of indicating conforming to any external label etc, not just ecolabels.
Re-using conformsTo is also better than minting a new rel term. conformsTo can be used in a wider set of situations that "ecolabel" in that it can be used to indicate non-ecological conformance to standards too. e.g. a web page can conform to HTML 4.01 Strict, or software could conform to "Certified for Windows Vista" or a toy could conform to a particular child safety standard.
This product conforms to <a rel="conformsTo" href="http://ecolabelling.org/ecolabel/certified-carbonfree/">certified-carbon-free</a>
This product conforms to <a about="#product" xmlns:dc="http://purl.org/dc/terms/" rel="dc:conformsTo" href="http://ecolabelling.org/ecolabel/certified-carbonfree/">certified-carbon-free</a>
These claims could be verified by the authority being linked to (in the case above ecolabelling.org) by simply providing a link back from http://ecolabelling.org/ecolabel/certified-carbonfree/ to the product:
as a poshformat on the certified-carbonfree page:
The <a href="http://foo.example.com/product" rev="conformsTo">Foo product conforms to certified carbonfree.</a>
as RDFa: by linking to it using an about attribute, a namespace URL declaration, and labeling it with rev="dc:conformsTo".