The provided tag "6551" doesn't appear to represent any common adult video tag. It is possible that it could be an internal identifier used by a specific website or content provider, but without more context or information, it's difficult to determine its meaning.
Porn video tags usually consist of keywords like fetish, kinks, sexual positions, body types, and other descriptors which help users find videos they are interested in. For example, common tags could include "lesbian", "anal", "facial", or "teen". If you can provide more context or information about the tag or website, I'd be happy to help clarify its meaning.