Sluggish Cognitive Tempo (SCT) is a descriptive term used primarily by researchers to refer to a group of individuals having attention problems that may be distinct from those seen in ADHD. These individuals are often characterized as being daydreamy, mentally foggy, easily confused, and staring frequently. They are also believed to have symptoms of hypoactivity, lethargy, slow movement, and even sleepiness.
SCT was originally discovered in the 1980s when the DSM-III first began subtyping ADHD as attention deficit disorder (ADD) with or without hyperactivity. Research comparing these subtypes produced a mixed pattern of results with some studies supporting the distinction while others did not. Later studies comparing these subtypes began to notice that those with ADD without hyperactivity seemed to have a higher frequency of symptoms of daydreaming, staring, being spacey or easily confused, mental fogginess, and hypoactivity or lethargy. By the late 1990s and early 2000s, studies began to appear that specifically chose participants for having these symptoms of SCT directly rather than those who met criteria for ADD without hyperactivity or ADHD-PI. Such research proved to be more fruitful in identifying a more reliable pattern of differences in cognitive abilities, comorbid disorders and impairments than had been the case for ADD without hyperactivity or ADHD-PI. Children with SCT appeared to have slow processing speed and reaction times, far lower rates of comorbid oppositional defiant disorder and conduct disorder, a higher occurrence of anxiety symptoms and even disorders, and possibly a greater occurrence of depression. Most consistent across these studies was a pattern of social withdrawal in interactions with peers rather than social intrusiveness, aggressiveness, and rejection so often evident in ADHD.

Their typically shy nature and slow response time has often been misinterpreted as aloofness or disinterest by others. In social group interactions, those with SCT may be ignored. Those with the other types of ADHD are more likely to be rejected in social situations, because of more intrusive or aggressive behavior.

http://en.wikipedia.org/wiki/Sluggish_cognitive_tempo

Isn't SCT a subset of ADHD/ADD?