Nikon D5100 Reference Manual page 27

The role of the Fn button can be selected using Custom
Setting f1 (Assign E/Fn button; 0 165), allowing the
following settings to be adjusted by pressing the Fn
(E) button and rotating the command dial:
Release mode (0 35)
ISO sensitivity (0 54)
Active D-Lighting (0 74)
Fn (E) button
Image quality/size (0 47)
White balance (0 81)
Auto bracketing (0 88)
+
Command dial
X
9