Refer to figure.
It shows the interface of an A320 FMS, including supplemental navigation data insertion and storage.

The navigation database cannot be modified; however, it has some additional storage space where the flight crew can create and store additional waypoints, routes, navaids, etc. they might need along their route. An example of an additional waypoint could be some Decision Points, which coordinates are indicated in the Operational Flight Plan the crew receives during the briefing. On a several sectors day, it is very common that the pilot stores the flight back (the second sector) on this additional storage of the navigation database. For instance, the FMS2 Honeywell Release 1A, allows the flight crew to create and store up to 99 waypoints, 10 runways, 20 navaids, and 5 routes. This will be different from one FMS to another, but it already gives you an idea of the capability of the additional storage space of the navigation database, called the supplemental navigation database.

Caution! It is really important to understand and make the difference that pilots can use the Supplemental Navigation database to write and read information => by adding their own customized waypoint or other navigation data. BUT this is limited to ONLY this Supplemental Nav database!

At the time of the Main Nav database update (every 28 days) the Supp Nav database is automatically erased. It makes sense as the Supp Nav database could include waypoints that have been defined using waypoints contained in the Main Nav database that may have changed as a result of the update.