Hi
To record fitness activities with the most accurate results, record them via the Workout app and be sure to choose the activity type that most closely matches your workout.
This enables your Apple Watch to use the most appropriate sensors and data sources when estimating results, including using tailored calorie calculations for certain activities.
To record workouts that are neither included within nor sufficiently similar to those in the main list - including the exercise class that you have described - choose "Other" as the activity type. This credits one minute of Exercise for each full minute of the workout and Active Calories (Move goal) at a rate equivalent to a brisk walk or based on data from the heart rate sensor, whichever is higher.
When ending the Other workout, tap the option to "Name Workout" to label the workout, choosing from a list of additional activity types including Mixed Cardio. The choice of label does not affect the recorded results.
The workouts that you do most often will be offered as Quick Start options within the Workout app (including Other workouts with their saved labels pre-selected).
More information:
Work out with your Apple Watch - Apple Support