To expand a little for those who are interested in the technicalities:

A home signal protects the entrance to a block, a "block" being a specified section of track. It can display two indications: Stop (red) when the block is occupied by a train, and Clear (green) when it it's not occupied by any trains. Both of these are referred to as "home indications". These concepts goes back at least to the 1870s/1880s, if not earlier (interlocking plants date to the 1840s).

A distant signal gives advance indication of the position of the next home signal. Since it doesn't govern the entrance to a block, all it can display is Caution (yellow) when the next home signal is in Stop position and Clear (green) when that signal is Clear. This also dates from the same era as home signals. Distant signals might or might not be controlled by track circuit(s), and when they are, they are referred to as "automatic distant signals".

What this means in three indication signaling (R-Y-G), which is almost universal today, is that each signal which can display three indications is a home signal for its block and a distant signal for the home signal at the next block. Three indication signals do not display D plates.

A series of blocks protected by automatic block signals is called ABS (automatic block system) regardless of the type of main track authority used: schedules, train orders TWC, DTC, interlocking, or CTC. All "automatic" means is that they are operated by the trains, with the exceptions of course of the controlled signals within interlockings and CTC, which have an additional manual control component.

Distant signals which actually display a D plate in this day an age are most frequently found in dark territory in advance of the home signal governing the use of an interlocking, such as a drawbridge or crossing another railroad at grade, and all they ever display is Caution (yellow).