As Dirtdives said and linked to, The Mantle is the formal rule set, like the Charter, of the group.
Additionally, there are also Terms of Service for all "Movie Props Sites LLC," the company that hosts and owns the 405th Forums:
You would do well to also read the Frequently Asked Questions, as they might not be "Rules" but they do explain how things work around here:
Hello, and welcome to the 405th Infantry Division, the world’s premier Halo Costume and Prop construction community. This document is designed to help answer some of the common questions new members have about our organization and its Forum or Message Board. First, what is the 405th? As...
Membership FAQ How do you become a member of the 405th? If you are reading this, then chances are good that you are in fact already a member! The 405th has several levels of voluntary membership where an individual can choose how much they want to be involved with the 405th and how much they...
“Official Events:” An Event is only an Official 405th Event if it has been approved by the Local Regimental Command Staff and has had a corresponding thread created in the appropriate Regimental Sub-Forum or the Convention and Prop Parties section of the 4054th Forums. Events may be requested...
And these are the rules for our Facebook Groups, which should also be fol owed here on the main Forum:
1 Be Respectful
Do not mock, belittle, or berate other members. We all start somewhere and we all have different skills. Seek to support, assist, and educate rather than put down.
2 Respectful Feedback
Members should be respectful and measured when offering feedback on Another user's build, when Feedback has been requested on the build.
3 Stay on Topic
Keep posts on topic (Halo Universe costumes and 405th Activities). Do not advertise unrelated items for sale or other groups here.
4 No profanity
405th Users and Members can be as young as 13 and as such this space should be kept Family Friendly.
5 No religion, politics, or discrimination
Any behavior that insults or discriminates anyone based on gender, race, orientation, etc. will not be tolerated. nor will discussion on Politics or Religion.
6 Personal Sale posts are allowed
Please include at least one photo, a brief description, and the price you are asking. Do not spam the page with sales or excessively bump your sales post.
7 Posts seeking Commissions of Armor are allowed
Include relevant information such as what Armor Style, from what game, sizing, material, budget, and location of requester.
8 No Streaming Advertising
No Twitch, Mixer, or Youtube Gaming channel advertising of any kind. Youtube videos and Twitch Creative are allowed if it is a specific tutorial on building Halo related props and armor
9 No Crowdfunding
No linking to or promoting of any Crowd Funding campaign or site such as Patreon, Go Fund Me, Ko-Fi, or others for Projects.
10 No Airsoft/paintball posts of any kind are allowed
We make costumes and props that are not intended to have any functional properties such as protection for airsoft, paintball, or other activities.