frozenEMT,
Usually when an officer or a Battalion Chief reports they are "using All Hands", a Recuperation/Rehabilitation and Care(R.A.C.) Unit will be dispatched.
A R.A.C. Unit is also assigned on every 2nd Alarm, 10-60(Major Emergency), 10-76(High-Rise Fire, Commercial) and 10-77(High-Rise Fire, Residential/Hotel). Any other time, I believe it is at the discretion of the incident commander.
Each R.A.C. Unit is staffed by a "Light Duty Firefighter".
R.A.C. Unit 1 - Engine 91(242 E. 111th St., Manhattan)
R.A.C. Unit 2 - Engine 332, Ladder 175(165 Bradford St., Brooklyn)
R.A.C. Unit 3 - Rescue 3, Collapse Rescue 3(1655 Washington Ave., Bronx)
R.A.C. Unit 4 - Special Operations Command(S.O.C.)(750 Main St., Roosevelt Island)
R.A.C. Unit 5 - Engine 242(9219 5th Ave., Brooklyn)