Use of “Negative” is quite common in Gerber files.
Traditionally the solder mask ls an inverted layer.
Here a header of one generated by KiCad:
G04 #@! TF.GenerationSoftware,KiCad,Pcbnew,5.99.0-unknown-99442350a4~142~ubuntu20.04.1*
G04 #@! TF.CreationDate,2021-10-26T22:38:27+02:00*
G04 #@! TF.ProjectId,mumar_base_stm32,6d756d61-725f-4626-9173-655f73746d33,rev?*
G04 #@! TF.SameCoordinates,Original*
G04 #@! TF.FileFunction,Soldermask,Bot*
G04 #@! TF.FilePolarity,Negative*
%FSLAX46Y46*%
G04 Gerber Fmt 4.6, Leading zero omitted, Abs format (unit mm)*
G04 Created by KiCad (PCBNEW 5.99.0-unknown-99442350a4~142~ubuntu20.04.1) date 2021-10-26 22:38:27*
%MOMM*%
%LPD*%
Apart from negative layers, negative features are also common. It is for example used to subtract pads from silkscreen, and KiCad’s Gerbview has a built-in menu option to make these visible.