Sony STR-ZA3100ES Operating Instructions Manual page 12

Parts Functions
 SOUND FIELD (2CH/
Selects the sound field you want.
DIRECT, A.F.D., MOVIE,
MULTI ST.)
  + (*)/–, 
Adjusts the volume level.
* The  + button has a tactile dot. Use the tactile dot as a reference
when operating the receiver.
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