What is the differentiation between "extinction optical depth" and "optical depth"?

[abnian@163_com]

Hello everyone!
In the program radsw.F90, there are two sentences like this:

'real(r8) :: aertau(pcols,nspint,naer_groups) ! Aerosol column optical depth
real(r8) :: aerssa(pcols,nspint,naer_groups) ! Aerosol column averaged single scattering albedo'

And in this program, there are also another two sentences like this:

'real(r8) tauxar(pcols,0: pver) ! aerosol extinction optical depth
real(r8) wa(pcols,0: pver) ! aerosol single scattering albedo'

Why “tauxar” and "wa" has no the dimension of vavelength (nspint)？
According to the books I have read, optical depth and single scattering albedo are the function of wavelength.
What is the meaning of "extinction optical depth"? What is the differentiation between "extinction optical depth" and "optical depth"?
Thank you!