From the official config with comments it is visible and documented:
/** Enable rescue system that helps hosts to reconnect and continue a print
* after a disconnect. */
#define HOST_RESCUE 1
/** What to do if power loss during print is detected.
* 0 = just stop where you are,
* 1 = raise z only
* 2 = go to safety park position
* Increasing value needs more remaining time with power. USV is required
* normally to be able to handle this.
*/
#define POWERLOSS_LEVEL 0
// Pin that signals power loss, -1 = off
#define POWERLOSS_PIN -1
// State to signal power loss
#define POWERLOSS_DETECTED 0
That is all related to power loss detection. Important for server is that HOST_RESCUE is set to 1.
