This is a list of used disc IDs and their effect:

Value Description Effect
'R' Revolution = Wii none
'S' Wii disc none
'G' Gamecube none
'U' Utility-disc(?) GameCube (GBA-Player disc is using U) none
'D' Gamecube Demo disc(?) none
'P' Gamecube promotional disc(?) none
'0' Diagnostic Disc auto boots title when disc ID is read by system menu
'1' same as '0', probably
'4' Wii Backup Disc unknown if any
'_' WiiFit Chan installer unknown if any