Hi PG,
When you have an analog to digital converter for say, a digital voltmeter, you have to be able to put the digital codes on to a display to give you the readout of the values.
The usual output code from a digital counter is to have 4 digital (0 or 1) outputs and the weighting of these is in a 1-2-4-8 progression. To count to 10 requires 4 bits, and these are usually denoted as D0, D1, D2, and D3. If the weighting of D0 is 1 and the weighting of D3 is 4, then the number 6 is represented by having Do = 0; D1 = 1; D2 = 1; and D3 = 0.
This code weighting can allow a number range of 0 to 15. There is another code weighting which is sometimes used and that is a 1-2-4-2 weighting; this gives a count range of 0 to 9.
Your original post asked about the D's and many of the answers referred to the actual hardware. My post is intended as complimentary to the prior posts.