Like the Hold Pedal controller, except this only sustains notes that are already on (ie, the device has received Note On messages, but the respective Note Offs haven't yet arrived) when the pedal is turned on. After the pedal is on, it continues to hold these initial notes all of the while that the pedal is on, but during that time, all other arriving Note Ons are not held. So, this pedal implements a "chord hold" for the notes that are sounding when this pedal is turned on. If a MultiTimbral device, then each Part usually has its own Sustenuto setting.
Note: When on, this also postpones any All Notes Off controller message on the same channel for those notes being held.
0 (to 63) is off. 127 (to 64) is on.