From the .FIT documentation, it looks like there are two floating point types defined, 0x88/float32 and 0x89/float64.
I can find no other reference to them in the documentation or Profiles.xls spreadsheet.
Are these two types simply IEEE-754 single- and double-precision floating point types? Are there any restrictions on valid data for them? The 0xffff "invalid" value is actually a valid quiet NaN value. Maybe this is by design.
Regards,
Mark