Canon Speedlite 90EX Instruction Manual page 19

Multi-Speedlite, Wireless Lighting Configurations
You can create two (A/B) or three (A/B/C) slave groups and set the flash ratio for E-TTL II
autoflash shooting (p.22 - 27).
Wireless flash with two slave
groups
(p.22)
A
Wireless flash with three slave
groups
B
(p.25)
C
A
B
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