Canon Speedlite EL-100 User Manual page 47

Autoflash Using Multiple Receivers
When the sender flash group is set to [ALL], all flash units fire at the
same output, and the total, combined output is automatically controlled
to provide standard exposure.
When sender flash firing is set to [Enable], the flash units in group A fire.
Fully Automatic Wireless Flash Photography
Use more receivers when more light is
needed, or as an easy way to ensure
ample lighting. Shoot as described
in "Autoflash Using One Receiver"
(p.44).
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