DownloadHelper and quality of YouTube videos

The DownloadHelper extension shows that many YouTube videos have multiple versions available, labeled as HQ18, HQ22, HQ35, etc. This blogpost explains which label covers which quality. The point is: HQ22 is the highest available.

P.S.: Actually there's a detailed listing of all these labels and their associated video resolutions in the preferences of the extension (at least in the current version).