According to José M.F. Moura, one of the IEEE Signal Processing Society, the term signal is considered as follows: "Ages ago, signal referred to some physical manifestation of information that changed with time and/or space. By signal we may still be referring to a physical manifestation but we might also be dealing with other symbolic or abstract information formats like a sequence of millions of the four symbols of the genetic code (the DNA bases A, C, G, T) arranged into genes and noncoding sections. Or, we may be referring to some other abstract attributes of sequenced information: cold, hot, high, low. Examples of signals include audio, video, speech, language, image, multimedia, sensor, communication, geophysical, sonar, radar, biological, chemical, molecular, genomic, medical, musical, data, or sequences of attributes, or numerical quantities; the list goes on." [Moura, 2009]. In terms of CDL-Flex reports and presentations, we consider signals as time-dependent variables representing process variables and having unique name, which can however differ in various engineering domains. The values of such variables can be measured in the real industrial plant, simulated by simulation models, loaded from a database or historian, entered via HMI, or obtained in other project-specific ways.

Moura, J.M.F., "What is signal processing? [President's Message]", Signal Processing Magazine, IEEE , vol. 26, no. 6, pp. 6, Nov. 2009, doi: 10.1109/MSP.2009.934636. [-]