A signal is a means of conveying information. An analog waveform and a digital edge are both examples of signals. The word signal, in this section, refers to the digital edge variety, also known as hardware signals. A terminal, on the other hand, is a named location where a signal is either generated (output or produced) or acquired (input or consumed).
When a terminal shares a name with a signal, it is not always clear which is being referred to—the terminal or the signal. The sample clock provides a good example.
Within most devices, there is a terminal such that the signal at that terminal is always used as the sample clock. So when you refer to the sample clock signal, you refer to this terminal. For instance, for M Series analog input tasks, this terminal is named the ai/SampleClock terminal. For analog output tasks, this terminal is named the ao/SampleClock terminal.
When you use the Timing function/VI to select the source of the sample clock signal for your analog input task on an M Series device, you choose a signal at some other terminal to act as the source for the ai/SampleClock terminal. In other words, NI-DAQmx connects your chosen terminal (a PFI terminal pin, for instance) to the ai/SampleClock terminal. Selecting the ai/SampleClock terminal as the sample clock source returns an error because a terminal cannot be connected to itself.