Similarly, all Diodes , and in particular, Varicap diodes are binned voltage controlled capacitors when reverse biased.
As such the exponents are 0, -1, -2 .. On a log scale a good varicap is linear and gives linear V vs f VCO characteristics but over a limited range, so the non-linearity is the 2nd order effects.
Polynomial Caps can have both negative and positive powers, when expressed as |integers| in a power series must be > 1
So think of it ;
Fixed, linear and nonlinear
as ; fixed, linear and Poly.
where linear is a 1st order
I think....