Protecting Photographs From Deletion - Nikon D3400 Reference Manual

Protecting Photographs from Deletion

Press the A (L) button to protect the current picture from
accidental deletion. Protected files are marked with a P icon
and can not be deleted using the O button or the Delete option
in the playback menu. Note that protected images will be
deleted when the memory card is formatted (0 201). To remove
protection from a picture so that it can be deleted, display or
highlight it and press the A (L) button.
A
Removing Protection from All Images
To remove protection from all images in the folder or folders currently
selected in the Playback folder menu (0 187), press the A (L) and
O buttons together for about two seconds during playback.
168 Playback and Deletion
A (L) button