Nikon D5500 Reference Manual page 107

Option
f 3D-tracking
e Auto-area AF
1
Display AF-area mode options.
Press the P button, then highlight the
current AF-area mode in the
information display and press J.
2
Choose an AF-area mode.
Highlight an option and press J.
Quickly compose pictures with subjects that
are moving erratically from side to side (e.g.,
tennis players). In AF-A and AF-C focus modes, user
selects focus point using multi selector
(0 85). If subject moves after camera has
focused, camera uses 3D-tracking to select
new focus point and keep focus locked on
original subject while shutter-release button
is pressed halfway.
Camera automatically detects subject and
selects focus point.
Description
P button
Information display
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