Simply put, it's a proposed new neurodevelopmental disorder, aka something similar to Autism or ADHD.
The symptoms are in some ways similar to ADHD, but in many other ways, the complete opposite:
slow movements,
lack of initiative,
over-cautiousness (in contrast to the impulsivity of adhd) and
a lack of mental and physical energy.
You can read more about it here:
https://en.wikipedia...cognitive_tempo
In previous variations of the DSM and ICD the symptoms were kind of considered a separate disorder from ADHD, but for various reasons it was baked into ADHD. However, Dr.'s eventually started realizing how different these patients were, from the hyperactives, especially in the fact that stimulant medication DOES NOT WORK, that a debate regarding re-evaluation of the symptoms started taking root.
It actually got so far that SCT only missed being added to the DSM-5 by a hairs smidgen - but, sadly, for various reasons, key ones being that the research was not as mature at the time, and that there was a great deal of pressure from ensurance-companies and politicians to NOT create so many new disorders, the diagnosis was not made official.
More recent research however, have truly started to hammer out that it's most likely a different disorder - mainly because all other ADHD-symptoms have been seen to recess when given various adhd-drugs, but not the SCT-symptoms - it's also now possible to diagnose, to categorise, the symptoms with different parameters and scales than ADHD - you can actually measure it now.
The symptoms of ADHD and SCT also correspond to different aspects of the neurological model for Attention - this also implies that there may actually be MULTIPLE other attention-disorders, other than these, potentially, two.
And, most damning, fMRI and other neuroimaging technologies have found differences in brain-anatomy between ADHD-ers and SCT-ers, showing that patients with SCT-symptoms show abnormalities in the BACK of the head - in the Superior Parietal Lobe and parts of the Default Mode Network - the patients with ADHD do not show these abnormalities, they show abnormalities in the FRONT of the head, in the Pre-Frontal Cortex. In fact, the abnormalities in the SPL are fairly unique for SCT.