What is the difference between "Route Planner" and "Course Planner" applets? And what does that mean in practical use? And if the differences don't matter, why are there two applets?
Short Answer:
The question above led to a long thread after I posted it in 2022. Since then, forum managers started pointing other threads here when they ask the same question. In summary, Garmin uses the vocabulary "route," "track," "course" and "activity" to describe what are all generally the same thing. Namely, each is an ordered set of 3D points (lat, lon. elev.) that defines a path relative to the Earth's surface. So the simplest way to understand them is to consider how they vary from one another.
Routes and tracks verses courses and activities
The difference between routes and tracks on the one hand and courses and activities on the other is the data format in which the points are stored. Tracks and routes are stored in a .gpx format, which is an easily transported universal standard. You can open a .gpx file in text software like Word or Notepad, and the data sort of makes sense. Courses and activities are stored in .fit format, which a Garmin proprietary format that is highly compressed (needs less storage space and makes no sense if you open the file in a word processor) and can store additional information like heart rate generated by Garmin devices that's not part of the universal .gpx standard. I'm nto sure, but I believe the compression is accomplished without loss of resolution.
Tracks and activities verses routes and courses
The difference between tracks and activities on the one hand and routes and courses on the other has to do with how they are created. If you define the ordered set of points explicitly, say when you're planning a hike, Garmin calls that a route or a course. If a Garmin device creates the set of points based on its location, then Garmin calls that a track or an activity. An obvious consequence is that points in routes and courses don't have timestamps, whereas Garmin devices timestamp the points the devices record.
The differences matter because (and only because) they affect how you manage your navigation data. In a perfect world, there would be only one word for all four concepts, and users would be asked to distinguish what they want when they export or import navigation data. But like most high-tech things, these terms developed over time. So possibly for backward compatibility, Garmin continues to use all four words to keep things simple for long-time users, even though that makes things painfully complicated for new users. In an attempt to minimize the pain, Garmin hides some of the distinctions, but that means users can be surprised and upset when what they've created is not what they meant to create. It also means that users can have a hard time finding their navigation data in a Garmin device if they don't understand how and why Garmin uses so many terms for what are all basically the same thing.
Caveat
I'm not an expert. So there are probably mistakes in the above. I hope they are minor and don't defeat the purpose of sorting out the main confusion.