Sometimes you cannot do the exercise as written. The rack is occupied, the gym does not have the machine, or your shoulder is sore that day. Substituting is how you log what you did instead, so the work stays on the record and your coach can see it.
How to log one
Post an exercise update as usual, turn on Substituted exercise, and enter the name of what you actually did in the exercise title. Record your sets and reps against that.
The hint on the toggle says the same thing: marking an exercise as substituted logs a different exercise without affecting your charts.
Why that matters
Your progress charts track one movement over time. If you log a leg press under the heading of a back squat, the chart shows your squat jumping by 100 kg. That jump then sits in the chart for as long as the chart exists.
Marking it substituted keeps the work on the record and leaves the trend alone. Your coach still sees exactly what you did, and the line for the prescribed movement stays comparable.
Choosing a substitute
Matching the movement pattern usually works better than matching the muscle. A barbell bench press swapped for a dumbbell bench press is close to the original. Swapped for a pec fly, it is a different exercise doing a different job.
If you are not sure, ask your coach rather than guessing. They set the programme, so they can tell you what fits it, and asking takes a moment.
When something hurts
If you swap a movement because it causes pain, tell your coach directly as well as logging it. Logging the swap is not the same as saying so. Pain that keeps coming back is a reason to stop and get it looked at.
Substituting every week
If you are swapping the same exercise out week after week, that is worth raising. Tell your coach, and they can prescribe something you can do with the equipment you have. It also stops the chart for that movement sitting empty.