Basically, from my experience, this is how I understand it.
If a route is created using the plan.tomtom.com website application, it can be:
1. Synced with all devices, including synced with your phone through the TomTom GO app = CarPlay,
2. and is able to be edited in plan.tomtom.com going to the three dots > Edit Route or Edit Description.
If a route is created through the TomTom GO app and saved to My Routes:
1. It doesn't show up on My Routes on your account through the plan.tomtom.com website application,
2. and cannot be edited (both route and description) within the TomTom GO app, it can only be deleted.
Moral of the story: Only plan and save routes using the plan.tomtom.com website and sync them to all of your devices, whether it be your TomTom navigation device or the TomTom GO app on your phone = CarPlay. Then at any time you can go back and edit the route, or its description, or delete it.
And another thing, routes created using the website planner cannot be deleted using the TomTom GO app.