Most of Workout Titan Coaching works the same way on every plan. The client limit is the main difference between plans. Beyond that, two features need a paid plan: building forms, and video uploads past the free allowance.
Forms
Building check-in forms needs a paid plan. On a free account the option says so up front and points you at the upgrade screen, rather than letting you construct a form and fail at the end. In Basecamp the same requirement is described more specifically as available on Performance and Infinity plans.
This is the difference most likely to matter, because recurring check-ins are how remote coaching scales. Without forms, the same job runs by message: asking every client the same five questions each week, and reading the answers spread through a chat thread.
Video uploads
A free account can upload 5 videos in total. Reach that and the upload option asks you to upgrade your plan to upload more videos.
Videos you have already uploaded are never removed, whatever happens to your plan. If a subscription lapses you keep your library. You simply cannot add to it beyond the free allowance.
External video links are not part of that allowance, so on a free plan they are the way to attach demonstrations without spending uploads. They carry their own trade-off: the video can be deleted or made private by whoever owns it.
What every plan includes
Worth knowing, because it is more than people assume. On Foundation you can still build unlimited workouts and exercises for your 3 clients, organize them into folders, attach videos to exercises, and message. You also get the calculators, body metrics and nutrition targets, weight and measurement logging, nutrition logs, habits, and every kind of notification.
In other words, the coaching itself is included on every plan. What a paid plan adds is more clients, plus forms.
Deciding whether to upgrade
Two plain triggers. The first is a fourth client, which is a hard limit rather than a judgment call. The second is finding you ask the same questions by message every week, which is the point where forms start saving you time.
If neither applies, Foundation is a full free plan rather than a trial, and there is no need to upgrade yet.