In terms of mathematic expression, in the equation of the second box, "alpha(f)" in the (minus) exponential expression simply says that alpha is a function of frequency f, but does not say what function it is (between alpha and f). If you want to use this equation, "alpha(f)" should be replaced by a value of alpha which is determined by the function of "alpha(f)" for a given value of f. Of course, you need to know what function it is before you can do so.
In the equation of the upper box, the exponential expression says that the the (minus) exponent is proportional to frequency f (in addition to the distance d). This can be viewed as a special case of the above equation with "alpha(f)" being a linear function of f, while the above equation is for more general cases.