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Please find responses for your query :

If you add a Participant Passcode to the meeting, participants will need the meeting ID along with Passcode to join meeting.

It is for participants to join meeting and for meeting security. 

It is your choice if you could add it or not while scheduling meeting. Without that also users can join (just with meeting ID). 

Moderator code is user account specific which is constant like your personal meeting ID and it is linked to your account. Moderator code is only for the host to join meeting to control meeting. It cannot be shared with participants. 

Thank you,