I took a look at the source code for this particular API. Apparently, Instructure themselves aren't exactly sure what this value means: "We *think* floating means 'future', or 'not submitted yet' -- Venk". Here's the commit where these comments were added: https://github.com/instructure/analytics/commit/033a70eb859f4d0b1a1b1720bb603420be7f6046. The commit that added the floating submission type unfortunately predates the open-sourcing of the Canvas analytics package, so there isn't really a way to figure out the context surrounding when it was introduced.

Looking at where Canvas determines the submitted status, I think the above assumption is correct, with the addition that unsubmitted assignments that have no due date will also show up as floating. Based on the way that code is written, floating seems to also be the catch-all for when no other status can be determined.