Captain Sim wrote on Feb 18^th, 2021 at 6:22pm:

The Captain_Sim\navigation\user folder designed for custom IAPs, SIDs and STARs. For example, if your custom files: KJFK_iaps.db KJFK_sids.db KJFK_stars.db are in the Captain_Sim\navigation\user folder, they will be used instead of main databases IAPs.db SIDs.db STARs.db for KJFK. You can modify files in Captain_Sim\navigation\user as you like without a risk of damaging the main database. Custom files can be used immediately after they are saved. No need to restart an aircraft, just press DEP ARR button and re-select the desired sid, star or approach in the FMC. Files in Captain_Sim\navigation\user won't be overwritten by navdada cycle updates.

:D Thanks for explanation, and more for actually including this function, that's way better than some other add-ons to keep my  customer airport alive.
BTW does it work for other dbs like Airports.db?