Three and a half digit display -
Compared to a full digit, a half digit can display only the number “1”. The half digit is always the first digit shown. Since the half digit is basically only a “1,” it has limited possible use.
A 3.5-digit display is actually four segments - one half digit and 3 full digits. Displaying with maximum capability, it would read “1999.” If we wanted to display 10kV on a 3.5-digit meter, we can make use of the leading half digit. In this case, we would have four digits of resolution with the meter displaying 1000. Placing the decimal point properly, the final meter reading would be 10.00 with the “kV” term screened on the front panel overlay
It can be used to show values from -1999 to +1999, and can include a decimal point, by lighting up the red segments, to suit the number you want
If you count the number of segment connections to each digit in the figure above, you'll see that the 1st has only 4 wires, because there are only 4 segments. The other digits have 8 wires (although we often refer to these as '7 segment' displays, if you include the decimal point, there are really 8 segments)
We say that the left hand digit is only half a digit, as it has half the number of segment connections of a full digit.
Significance of three and a half digit display -
3.5-digit (or 4.5-digit or X.5-digit) displays are primarily used in applications where the readings are limited to X digits, and where the full reading will not go above 1999 or 3999 or (X-1)999. It also ensures that the resolution is used up efficiently (only with respect to compatible values). Using a full digit display for an application where only 3.5- or 4.5- or X.5-digit display is required would increase the cost and disrupt the resolution
Moreover, by adding a relatively low cost display to the system the manufacturer doubles the displayed range. The X.5-digit display does not affect the accuracy, but it does affect the displayed apparent resolution.