&fmt=18 is video with _480x360 pixels resolution (_4:3 ratio) recorded at _512 kbps
&fmt=22 is video with 1280×720 pixels resolution (16:9 ratio) recorded at 2000 kbps
&fmt=35 is video with _854x480 pixels resolution (16:9 ratio) recorded at _900 kbps
&fmt=18 is what’s considered high-quality
&fmt=22 is the standard for high-definition
&fmt=35 is the true version of high-quality.
via What are the differences between HQ18, HQ22, &, HQ35 standards? – Yahoo! Answers.
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