The SUD Level reading - short for subjective levels of disturbance (previously discomfort) - is simply ascertained by asking someone, "On a scale from 10 - 0, 10 being the highest possible level of disturbance, and 0 being no disturbance at all, where would you say you are right now?"
This is used in EFT treatments to give the practitioner and the client a conscious awareness and way to know if there have been any changes in the feelings as the round progresses, and/or from one EFT treatment round to the other.
Please note that the SUD scale ends with the "zero point of peace". In order to complete a full EFT treatment, one needs to go beyond that into POSITIVE emotions. See also Energized EFT.