A3: Built-In Af-Assist Illuminator; A4: Rangefinder - Nikon D5600 Reference Manual

a3: Built-in AF-assist Illuminator

Choose whether the built-in AF-assist
illuminator lights to assist the focus
operation when lighting is poor.
Option
The AF-assist illuminator lights when lighting is poor (for
On
more information, see page 342).
The AF-assist illuminator does not light to assist the focus
Off operation. The camera may not be able to focus using
autofocus when lighting is poor.

a4: Rangefinder

Choose On to use the exposure indicator to determine whether
the camera is correctly focused in manual focus mode (0 95;
note that this function is not available in shooting mode M, when
the exposure indicator instead shows whether the subject is
correctly exposed).
Indicator
G button ➜ A Custom Settings menu
Description
G button ➜ A Custom Settings menu
Subject in focus.
Focus point is slightly in front of subject.
A Custom Settings: Fine-Tuning Camera Settings
AF-assist illuminator
Description
237