Great questions. Let me answer them in reverse order.

- There is no one-to-one assumption between variables. Multiple data variables 
can use a single uncertainty variable and multiple uncertainty variables can be 
linked to a single or multiple data variables.
- The units for the uncertainty do not need to match the data variables. If the 
units attribute is listed then the uncertainty is in those units, else if no 
units attribute then the units match the data variable. The data provider would 
need to ensure units are clearly understood.
- I think your question about velocity vs speed & direction is dependent on how 
the uncertainty is presented. If the uncertainty is a velocity then the units 
should (would?) match the data velocity. But if there is need to present the 
uncertainty of speed and direction independently that is possible. For those 
the units attribute must be defined.
- I see no reason why the uncertainty could not be a percentage of a 
measurement as long as it's clear how to use that uncertainty description. I 
would see providing the uncertainty as a percentage as requiring a formula to 
convert from percentage to data units. So there would need to be a formula 
attribute to describe how to convert the values to data units.

Does this help?

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