A blank subtraction removes the instrument background from the measured reading, leaving the net signal attributable to the sample alone. The instrument reads 18 signal units and the blank is 6, so the net signal is 18 - 6 = 12 exactly for the stated toy calibration ledger; real instruments have blank instability, nonlinear response, sample matrix effects, and measurement uncertainty.
A sample signal is not accepted until the blank is removed. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A measured signal still includes background
The instrument reads 18 signal units before the blank is removed.