B303: Found on a blog, somewhere, someday

A redirection status, used to indicate the linked resource should exist, or will exist, or indeed may already exist but one is too lazy to link to it. When you need a link but don't know what, use this page, then fully intend to optionally put the real link in the comments.

This is different to the B04 error, which indicates authoritively, in no uncertain terms, that the requested blog indeed does not exist.