Which comes first, the “chicken or the egg”?
When you are planning an IFR flight, it’s always a good idea to file your flight plan with ATC before actually entering it into the FMC. In the days when X-Plane had a far less capable ATC system it did not matter much when you filed a flight plan in the ATC window,. You would get it from the FMC. Now that SIDs and STARs are accepted by the X-Plane 12 ATC system, if you are entering the flight plan directly into the FMS first, there is a 50/50 chance that ATC will not accept the flight plan that you proposed. Maybe your desired runway is not in operation today (weather or some other reason) or maybe a waypoint is not recognized. Then you have to go right back to the planning stage and change things around to a greater or lesser extent.
If you first put in your SID, waypoints and STAR and ATC accepts them, then you can go to the trouble of entering it all into the FMS afterwards. If you are loading your FMS from the output of your flight planner and then filing it with ATC, you run into the same problem.
While not so convenient in X-Plane because you are a “one man band”, in real aviation the flight plan is actually entered into the FMS as late as possible – certainly after the flight plan has been filed. So this tip brings us a step closer to real aviation.