Dear @mathiasbockwoldt 

Thanks for raising this. I am glad to learn that udunits has been updated! 
Since CF1.8 gives https://www.unidata.ucar.edu/software/udunits/ as its 
reference for udunits, which means udunits 2 in effect, I agree that the text 
should be changed. We need to agree a specific change. Quoting a bit more of 
the current text:

> The Udunits package defines a few dimensionless units, such as percent, but 
> is lacking commonly used units such as ppm (parts per million). This 
> convention does not support the addition of new dimensionless units that are 
> not udunits compatible. The conforming unit for quantities that represent 
> fractions, or parts of a whole, is "1". The conforming unit for parts per 
> million is "1e-6".

I would suggest we replace that with

> The Udunits package defines a few dimensionless units, such as percent and 
> ppm (parts per million). This convention does not support the addition of new 
> dimensionless units that are not udunits compatible. The conforming unit for 
> quantities that represent fractions, or parts of a whole, is "1".

That is, we remove the recommendation to use 1e-6 for ppm. What do you and 
others think?

I am going to change the label of this issue to Enhancement because I would say 
that this is actually a material change to the convention (although a minor 
one) and thus deserves more scrutiny than a defect correction. 

Cheers, Jonathan


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