Canon SPEEDLITE EL-1 Advanced User's Manual page 149

Caution
When setting the flash mode to <
Speedlites supporting auto external flash metering. Firing in this flash mode is not
possible if auto external flash metering is not supported.
When the flash mode is set to <
a single group to obtain the standard exposure for the main subject.
Note
For < >, see the Instruction Manual of the Speedlite that supports Auto
external flash metering.
The order of firing among firing groups does not need to be consecutive; for
example, A, C, E can be set.
If there is a group that you do not wish to fire, set it to <
flash mode in Step 3.
>, make sure the receiver units are
> < >, exposure is controlled as
149
> when setting the