I found this interesting article:
https://documentation.meraki.com/zGeneral_Administration/Firmware_Upgrades/Meraki_Firmware_Release_P...
The bit of interest is:
"A stable release candidate matures into a stable version over time as it is slowly rolled out to devices globally. After 20%+ of devices are running the stable release candidate, a final formal review is conducted on the firmware. Again the same KPIs are analyzed as used in the stable release candidate review. Upon completion of these processes the firmware can be promoted to "Stable". After promotion, stable versions can be applied by any customer via the firmware upgrade tool on dashboard. The latest stable version is also the version that is used for all newly created dashboard networks for a particular device. "
So in short, 20% of the devices have to upgrade to the stable release candidate before it can be considered for getting the label of "stable".