The transmittion of a 10-75(Working Fire) equals a total of 4 Engines, 3 Ladders(1 for F.A.S.T.), 2 Battalion Chiefs, 1 Division Chief, 1 Rescue, 1 Squad, and usually 1 R.A.C. Unit on the scene of a fire.
Usually, there are 3 Engines, 2 Ladders, and a Battalion Chief on a Box. When the 10-75 is transmitted, an additional Engine, an additional Ladder(F.A.S.T.), and an additional Battalion Chief are added to the initial assignment, with the further additions of a Division Chief, a Rescue, and a Squad.
Rescues and Squads respond on all 10-75's, if available.
Also, if a Squad is 1st due to a Box, and is already assigned, another Squad will be assigned on the Box. For example, if Squad 18 is first due to a fire, and the 10-75 is transmitted, Squad 1 out of Brooklyn will be the 10-75 Squad. However, with Rescues, if a Rescue is already on the Box, I don't believe an additional Rescue is on the 10-75, but again, I'm not certain.