Sony UWP-D27 2-Person Camera-Mount Wireless Omni Lavalier Microphone System (UC90: 941 to 960 MHz) - Manual (Page 227)

The FEL lock cannot be set when a flash is not attached.
If a flash that does not support the FEL lock is attached, an error message is displayed.
When both AE and FEL lock are locked, (AE lock/FEL lock icon) lights up.
Related Topic
Assigning frequently used functions to buttons (Custom Key)
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