Also from: BUSS, Backup Altitude Scale
A backup speed scale and a backup altitude scale replace the normal speed and altitude scales when all three ADRs are switched off. Airbus recommends the BUSS not be engaged above FL250. Above that altitude the loss of speed data will most likely be short-lived (due to icing).
The speed information is based on angle of attack and depends on slat/flap configuration.
Altitude information is the GPS altitude.