A selector responds to a training mass-to-charge ratio. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Selection uses a ratio, not mass alone
The first training ion has mass index 12 and charge index 3, so the ratio is 4.
m/z=12/3=4
Another mass can have another ratio
The second ion has mass index 18 and the same charge index 3, so its ratio is 6. These are dimensionless training indexes.