To over simplify things...
You can look at it as, both fighters start with 10 points.
- The fighter that loses the round, loses a point
- The fighter that gets knocked down in a round, loses a point.
- Other than that, a fighter can lose a point for doing something illegal, at the refs discretion.
Rounds without Knock downs, or referee point deductions, normally end with the score of 10-9.
Now, if you wanna talk about, how a judge is supposed to assess who wins a round, thats a pretty detailed discussion.