Hi
Functional Training is explained here:
https://en.wikipedia.org/wiki/Functional_training
In the Workout app on Apple Watch, Functional Training is one of a number of labels that can be applied to workouts that are recorded under the "Other" activity type.
The choice of label for Other workouts has no effect on how the results of the workout are calculated.
To record the most accurate results for workouts on your Apple Watch, 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 workouts.
To record workouts that are neither included within nor sufficiently similar to those in the main list, choose "Other" as the activity type.
For "Other" workouts - regardless of the label, if any, that you select for them - Apple Watch credits one minute of Exercise for each full minute of the workout and Active Calories at a rate equivalent to a brisk walk or based on data recorded by the heart rate sensor, whichever is higher.
More information:
Use the Workout app on your Apple Watch - Apple Support
Use the Activity app on your Apple Watch - Apple Support