Nikon D3 - Manual (Page 219)

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Modeling Illumination
CLS-compatible Speedlights such as the SB-800 and SB-600 emit a
modeling flash when the camera depth-of-field preview button is
pressed. This feature can be used with Advanced Wireless Lighting to
preview the total lighting effect achieved with multiple flash units.
Modeling illumination can be turned off using Custom Setting e3
[Modeling flash] (pg. 326).