The Semester Unit is legally called "Credit Hour."
The different between the Clock Hour System and the Semester Unit System is that institution needs to keep track record of every hour under a Clock Hour System, and it has no regular end of course. That is, professors have a harder time moving on and teach new course when there are students still behind because of excess absence. However, a Clock Hour System can ensures that everyone passes if they take sufficient amount of time and money, and passing the quality assurance.
Another difference is the minimum duration. On a Clock Hour System, an Academic Year needs to be nominally at least 26 weeks which allows optimized institution to have 2 Academic Years in a single Calendar Year. The Semester Unit System requires a minimum of 30 weeks which limits the pace of academic acceleration since there are 52 weeks in a year, closer to 313 weeks every 6 years (look up 52/53 week fiscal year).
Block Scheduling for higher education. The regulation requires 10 minutes of break for each full consecutive academic hour after the first academic hour.
A Carnegie Unit is established as 1 clock hour per day, 5 days per week for 24 weeks = 120 clock hours = 144 academic hours. The Carnegie Unit is used for Secondary Education. A note is that the Carnegie Unit does not expect students to work outside of class.
Academic Year = 900 Clock Hours = 24 Semester Units
Clock Hour = 60 nominal minutes = 50 real minutes
Semester Unit = 37.5 Clock Hours = 45 Academic Hours = 2250 nominal minutes = 1875 real minutes.
Quarter Unit = 2/3 Semester Unit = 25 Clock Hours = 30 Academic Hours = 1500 nominal minutes = 1250 real minutes.