For each exercise you add to a workout session there will be an optional text field that you can fill out to add additional instruction for coaching your clients. Here are a few examples of how we've seen this used by our customers:

• Writing in RPE (rate of perceived exertion): 0-10 scale used to recommend or measure the intensity of an exercise (source).  
• Writing in rest periods: Period of time for your client to rest between sets.
• Writing in tempo: Four numbers that constitute the tempo of an exercise (source), so it may look something like this: 3010
• Writing in seat settings for specific exercise equipment.
• Writing in extra instructions for performing exercises that require guidance such as EMOM, Crossfit style supersets, cardio exercises where intensity changes over time, etc. 
• Writing in motivational notes for your clients to follow if you're sharing the workout with them.

When you add a note the field will display clearly for you to utilize while tracking the workout, whether you're using the web or mobile apps.