The navigation part identifier is basically a mechanism to tell eZ publish which "admin tab" you're currently inside. This is used for showing the correct design in the admin interface.
The following navigation parts are defined in eZ publish:
The following override keys are available for content/view templates:
object: id
node: id
parent_node: id
class: id
view_offset: integer
navigation_part_identifier: string
viewmode: string
depth: integer
url_alias: string
section: id
From 3.4 we also have class_identifier override so you don't need to use the class id when you would like to do an override.