Units spectrum x-ray

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).

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