A cohort needs somewhere to keep its gold that is not one member's purse, and a shop needs stock that the shopkeeper does not have to stand next to all day. A treasury is a chest on the property spine that belongs to a group rather than to a person, and it is where the coin and the goods of an organization live.
How it works
You claim a treasury the way you claim anything else. Press E on the chest, then say what it is: a vault, which holds gold and items for its owner, or a shop stock, which holds goods you have priced for sale.
Ownership can be a character, a cohort, a jarldom or a faction, so a clan treasury outlives whoever founded it and a hold treasury belongs to the seat rather than to the Jarl personally. Assigning a business or vault owner is the Jarl's inside their own hold, and staff can reassign one when a group falls apart.
Deposits are open and withdrawals are not. Anyone may put gold into a treasury and only somebody with control over it may take gold out, which is what makes a treasury usable as a dues box, a war chest or a tithe.
A shop stock is a treasury with prices on it. You set a price for an item and a maximum you are willing to sell, and remove the listing when you are done. Buyers open the chest.
Locks and keys still apply. A treasury is a chest, and who may open it is decided the way it is decided for every other container (see Keys, Locks, and Locksmithing).
Commands (/help treasury)
/treasury list lists the vaults and shops you own.
/treasury info <id> inspects a treasury's owner, chests, gold and inventory summary.
/treasury claim <vault|shop_stock> <hold> [name] claims the chest you last pressed E on as a treasury.
/treasury open <id> opens the vault or shop chest to deposit, withdraw or buy.
/treasury deposit <id> <gold> deposits gold. Anyone may deposit.
/treasury withdraw <id> <gold> withdraws gold from a treasury you control.
/treasury price <id> <base-form-id> <gold> [max] sets a shop price.
/treasury unlist <id> <base-form-id> removes a shop listing.
/treasury setvault <id> rebinds a controlled treasury to the chest you last pressed E on.
/treasury setowner <id> <character|cohort|jarldom|faction> <owner-id> assigns a business or vault owner. The hold's Jarl, inside their own hold.
/vault is an alias for /treasury. Personal coin is a different house (see The Public Bank).