What I wrote is not completely correct for DarkTable.
This image has gray values of 0%, 25% and 50%:
Exposure correction of 1 will change this to 0%, 50% and 100%:
Brightness adjustment of 1 produces 0%, 85%, 93%:
It seems that in brightness adjustment complete black is not affected (in contrast to Gimp), while complete white can not be reached (even with multiple instances). In any case, exposure and brightness adjustment produce very different results in Darktable.