Software reset
From Sega Retro
Most software produced for the Sega Saturn and Sega Dreamcast have a sofware reset feature. As the name suggests, this involves "resetting" the "software", returning the user to an earlier state, though is implemented by software recognising button combinations, rather than through a physical switch on the hardware.
Contents
Saturn
The software reset was mandated by Sega for the Saturn[1] and is performed by pressing 
+
+
+ START simultaneously. Saturn games should be able to respond to the software reset at any point (save for when memory operations are occurring[1]) and return the player back to the title screen[1]. If the player is already on the title screen, the software reset should send them back to the BIOS (or "audio CD control screen" as it was also called)[1].
Any configured options must be retained during a software reset[1].
Dreamcast
The software reset was also mandated by Sega for the Dreamcast except for "cases of special processing" (memory operations)[2]. Here the command is ++
+
+ START through regular controllers, however variations exist for other controllers[2]. Like the Saturn, the soft reset should return the user to the title screen, or if he/she is already at the title screen, to the BIOS[3].
For the Dreamcast Keyboard, the software reset is performed with Ctrl + Alt + Delete [4]. The Dreamcast Gun requires ++ START [3].
The software reset is especially important with the Dreamcast as a physical reset button does not exist on the unit. This means the hardware can only be reset by turning the power on and off.
Other systems
It is not thought that supporting the software reset became part of Sega's development guidelines until the advent of the Sega Saturn, however some Sega Mega-CD games are known to implement one using the same +++ START combination. In these cases, the player is always taken back to the title screen (or a similar point at the beginning of the game); there is typically no means of accessing the BIOS without resetting the console.