The new schema.org by Google, Yahoo and MS recommends usage of the attribute to display the status of products in an online shop:
&
The specification as of March 2020:
If the
relattribute is used, the element can only sometimes be used in thebodyof the page. When used with theitempropattribute, the element can be used both in theheadelement and in thebodyof the page, subject to the constraints of the microdata model.
which is the opposite to what the accepted answer says as far as the rel attribute is concerned. I have personally never experienced issues by adding rel in the head though.
Microdata is a WHATWG HTML specification used to nest metadata within existing content on web pages. Browsers, web crawlers and in particular search engines can extract and process Microdata from a web page and use it to provide a richer browsing experience for users.