Here is a minimal lesson file:
header {
    countin_perc = compareintervals
    title = "Compare intervals"
    lesson_id = "9f830e12-1f50-4fa9-8688-1e04469692fa"
}
This file will make an exercise that ask you to compare harmonic intervals. And since you do not say which intervals, it will ask for all intervals from a small second up to a major decim.
first_interval_type, second_interval_typeLet you select if the intervals you are asked to compare should be a
    melodic or a harmonic interval. The default value is
    melodic. Possible values:
    harmonic and melodic.
    
    first_interval_type = melodic
    second_interval_type = harmonic
    Modules: compareintervals.
first_interval, last_intervalSelect which intervals to select from when creating the questions. This variable should be defined the same way as ask_for_intervals_0. If these two variables are not defined, then the user will be able to select which intervals to practise from the Config page of the exercise.
Modules: compareintervals.