I generated this fluence spectrum and I wanted to understand the units of measurement with which FLUKA gives me the values, thanks for the answer.

I guess this is a USRBDX (or USRYIELD) output referring to a region boundary, i.e. a scoring surface.

If you input in the respective scoring card the area (in cm^2), what you get is *the differential particle fluence averaged over the surface, in GeV^-1 cm^-2 per primary particle*. If you did not, you can always divide by the area afterwards and get the same quantity.

In case of current (rather than fluence) scoring, the energy spectrum **integral**, not divided by the area, represents the number of particles crossing the surface (per primary particle), which is an a-dimensional quantity.

Remember instead that fluence [cm^-2] means tracklength density, namely travelled path [cm] per unit volume [/cm^3], which is a meaningful physics quantity to characterize the radiation field (see the didactic material).