What are D_b and D_a in the Generalized Fugacity Coefficients table?

I've been referring the book: Chemical Process Principles: Part-II Thermodynamics. I found a table that gives me fugacity coefficients in a tabular form corresponding to various reduced pressures and reduced temperatures. This is it:


![enter image description here](https://i.stack.imgur.com/n6nqa.jpg)


I dont know what the $D\_b$ and $D\_a$ in the table. In the bottom of the table, I see:


$$\left(\frac{f}{p}\right)^{'} = \left(\frac{f}{p}\right)(10^{D({z\_c}-0.27)})$$


Is the $D$ in this equation related to the $D\_b$ and $D\_a$ in the table?




$D\_b$ and $D\_a$ are deviation terms which must be used in the place of $D$ to obtain the fugacity coefficients for fluids whose critical compressibility factor is not 0.27. Replace $D$ with $D\_a$ if $z\_c > 0.27$ and with $D\_b$ if $z\_c < 0.27$.


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