The tool tip tells you that the module will accept linear or non linear data and the its actions are not linear and its output is non linear. The pipeline works in stages and each stages or profiles has a color space which defines the primary colors, WB and transfer function to allow for correct colorspace converstions and display. Linear just means that a gamma of 1 was used, sRGB is a specific color space and its not linear.it uses a gamma of 2.2 generally by definition. The role of filmic and issues around the pipeline and linear parts of the workflow are well discussed here Darktable 3:RGB or Lab? Which Modules? Help!
if you have not seen this and basically I find this the best way to see what filmic is doing to the image
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