I use it so that all my managed MT devices can be reached easily through the trusted subnet/vlan. I wouldnt attach it to the WAN interface, more common as a default is the LAN interface. https://help.mikrotik.com/docs/display/ROS/Neighbor+discovery If u check on the photo, What actually does it do ...

hi, just new here.

if I set Interface list named WAN to my ISP interface.
then in IP> NEIGHBORS DISCOVERY SETTINGS to "WAN" with the tick box.

What actually does it do WITH the tick box and WITHOUT the tick box?

I am trying to hide MT on the internet/ISP, but im lost. TIA