In Deciphering (1) I posted a recent version of Line 21 which runs from Port de Gentilly to Gare Saint-Lazare. I also sliced the map to highlight a couple of individual stops along the route. The stops shown in that slice are available in **both** directions.
Here’s a slice of that same bus line for stops that **do not** necessarily run in both directions. The <<Berthollet-Vauquelin>> stop on Rue de Berthollet only takes passengers going in the Stade Charléty direction. If you want to go in the opposite direction, your bus stop is on Rue Cl Bernard.
The other two stops indicate they take passengers in both directions, but you need to be at the bus stop going in the correct direction (right or left side of the street). There are several additional useful visual “clues” to assist you along with me personally!