You can't SUBTRACT the analog light values, but you can take a 'proportion', or split off a ratio proportion.

So, with an amplitude value of '142' as example, that arriving light could be split, into ten paths, each being

'14.2', as evenly as possible, and 1 of those paths diverted, over (laterally, or X10) into the next power of ten column, or 'macro-column'.

This action gives a multi-digit multiply approach resembling regular 'carry-over' as the early school math teaches, to carry a 'tens digit' over, after a partial multiply. Doing, say, 9 X 9 = 81 suggests sending the '8' portion over...; over to the left to be tabulated in that 'tens' column.

In this case, optical BUS, the carry-over could be 10 %

and unconditional, but would also do carry-over of a number like '8', where the action is to carry over a '0.8' to the next higher macro-column, while leaving 7.2 n the one's column.

Did I mention this shii can be confusing

The expected actions would have been, to keep all of the (light quantity) in the first, lowest column, the 'ones' column. But the total quantity is not lost, it is just moved over and gets totaled in next column over

Now, as to magnitudes, the lowest weight column is physically separate, and does not match, physically, being 10X too big, in magnitude. The consequence of this is that a carry-over, like the '8.1' in '81' can be easily put through a divide-down, by 10.

Actual is 0.9 X 0.9 = 0.81 and so the actual carry-over is 1/10 or 10%, and that's = 0. 081 literally. That sizing fits perfect into next column ACTUAL amplitude, as a carry, albeit a little different from a purely single digit, it's expected to be, '0. 08'.

Not really that confusing, just new...The total light quantity is simply done differently, but result or 'summation', across laterally, across the bottom of column, should always be same.

This method is shown, in a few recent posts, where a number like '0. 184' is expressed as '0.1', '0.8', '0.4'.

That's an amusing variation, of course needing to keep the three separate, and need to attenuate the low digit, when doing any full summation.