Canon Speedlite EL-5 Advanced User's Manual page 94

Group Control
You can add receivers when more light or sophisticated lighting is needed. For added
receivers, simply specify the firing group (A, B, or C) that you want to be brighter.
For example, when three receivers are set to firing group <
a single, high-output Speedlite in group A.
Caution
To fire the three groups of A, B, and C, set to <
when Speedlites are set to <
Aiming the units in firing group C directly at subjects may cause overexposure.
Note
Flash ratios from 8:1 to 1:1 to 1:8 correspond to 3:1 to 1:1 to 1:3 (in 1/2-stop
increments) as converted to number of stops.
Firing group A
>.
94
>, they are all controlled as
>. Group C does not fire