HIIT is high intensity interval training or interval training in which the person exercises to at least 80% of his maximal heart rate and often to 80-90% SIT is sprint interval training in which one goes all out exercising to the maximal possible.MICT ( moderate intensity continuous training) is the traditional aerobic exercise training in which the person does not get much over 50-60% of his maximal oxygen uptake.
There is not much really new to HIIT or SIT other than the names. Roger Bannister did not invent the technique but he certainly used it in the preparation for his race ( 1954) that broke the four minute mile. He would run ten quarter mile intervals in 59 seconds each jogging in between each. Today his training might be characterized at high intensity , low volume.
An excellent review on these 3 types of training and the important effect of exercise intensity is found in a review by Maclnnis and Gibala (1) Gibala is Chair of the Department of Kinesiology at McMaster University and has written extensively about high intensity interval training.
The authors review aspects of the three training modes on mitochondrial proliferation,increases in capillary density and increases in V02 max. And the winners are for two of the three are SIT and HIIT.Only in regard to increases in capillary density brought about by training does MICT hold an advantage but even that is in dispute as some studies show HIIT is just as good.
In regard to all three it seems that the data strongly suggest that the key variable is intensity not duration nor frequency of training.See here for a 2016 lecture by Gibala outlining the case of interval training and the argument that intensity is the key variable.
Interval training is no longer just inside baseball type talk among athletes and coaches but has becoming mainstream in the world of rehabilitation , for example in cardiac ( see here) and pulmonary patients( see here), type 2 diabetes (see here)and heart failure patients.
(1) Maclnnis MJ and Gibala,MJ Physiological adaptations to interval training and the role of exercise intensity.J. of Physiology 2016,October 17