Canon Speedlite 90EX - Manual (Page 28)

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If you need more flash output or want to perform
advanced lighting, you can increase the number of
slave units. Set an additional slave unit to the firing
group (A, B or C) whose flash output you want to
increase. There is no restriction on the number of
units.
For example, if you set a firing group with three slave
units to <1>, the three units are controlled as a single
firing group A with a large flash output.
About Slave Group Control
ID = A ID = A ID = A
Firing group A
COP
Y