The measured number is called the peak gain at zero degrees viewing axis.
It represents the gain value for a viewer seated along a line perpendicular to the screen's viewing surface.
The gain value represents the screen's brightness ratio relative to a set standard (in this case, a sheet of magnesium carbonate).
Since a projection screen is designed to scatter the impinging light back to the viewers, the scattering can be highly diffuse or highly concentrated.
Highly concentrated scatter results in a higher screen gain (a brighter image) at the cost of a more limited viewing angle (as measured by the half-gain viewing angle), whereas highly diffuse scattering results in lower screen gain (a dimmer image) with the benefit of a wider viewing angle.