I would assume it's a ratio of masses, and therefore is a dimensionless value.
Ok, but the value is expressed as a percentage, or not?
For example in my study the maximum value of tracer mass fraction is 0.0877.
This is 0.0877 % or 8.77 % ?
A ratio typically has the range 0-1, where 0 = 0% and 1 = 100%