if you go to form 19 you will see station dial 0 routing and priority dial 0 routing, in these fields you can direct a user who dials 0 to different locations. One example and there can be many is:
all users are in the same tenant group but you want to identify a call from the President of the company who dials 0 from the rest of the staff, you can programm a different key on the console in the priority dial 0 field for him/her and then in the President's cos add option 239. When the President dials 0 and say goes to the console she will know it is him from the key location on the console. There are lots of scenarios for this feature.
Mailbox dial 0 follows the 0 routing of the tenant of the voicemail, if the voicemail ports are programmed as tenant 6 then in form 19 tenant 6 you would program dial 0 routing there.
Hope this helps.