You can use steemdb.com to view the raw data of a post and explore the different ways the JSON metadata is used by clients. For example, here is a link to the raw data for your comment. Scroll down to the json_metadata field. Notice that ChainBB reported the app/version in the app field. Also a field specifying the format of the content and an array of tags for the comment are also included. There isn't any official documentation for how JSON metadata should be used (it is not like it is enforced by the blockchain anyway), but various unofficial standards seem to have emerged, mostly by following the usage led by the steemit.com client.

There is a draft spec available for the Communities feature in the wiki of the Steemit Condenser repository. Keep in mind that this is just a draft and things may change with the design.