Using plus sign in custom internet media types (MIME types)
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I'm building a RESTful web service, and I'm creating some custom media types. It seems like in media types the subtype becomes more specific as you go right to left using '+'. So for example:

`application/json` is more general than `application/hal+json`, since [HAL](http://stateless.co/hal_specification.html) adds more restrictions on top of json.

My question is whether this is standard, or just a convention (I've poked through some of the RFCs on MIME types and I couldn't find anything, but I may not have looked hard enough).

If it is standard, does it generalize so I can add even more specific subtypes like `application/vnd.custom.mime+hal+json` or is it limited to one "+"?