I personally read the term Digital Number (DN) only when using optical data, but I guess it’s kind of the same:
DN is the measurement before calibration while reflectance is the proportion of radiation hitting a surface to the radiation reflected away from it (as for optical data).
DN can be compared with amplitude A, the raw format of backscattering. A² is intensity I, still not calibrated but the proportion of microwave backscattered from that area on the ground. To get comparable values of SAR backscatter, calibration is needed. Calibration corrects intensity according to sensor specifications and (local) incidence angle.
Beta Nought (ß) is the radar brightness (or reflectivity) coefficient. The reflectivity per unit area in slant range is dimensionless. This normalization has the virtue that it does not require knowledge of the local incidence angle (e.g. scattering area A).
Sigma Nought (σ), the backscattering coefficient, is the conventional measure of thestrength of radar signals reflected by a distributed scatterer, usually expressed in dB. It is a normalized dimensionless number, which compares the strength observed to that expectedfrom an area of one square metre. Sigma nought is defined with respect to the nominally horizontal plane, and in general has a significant variation with incidence angle, wavelength, and polarization, as well as with properties of the scattering surface itself.
Gamma (γ) is the backscattering coefficient normalized by the cosine of the incidence angle.