I will attempt to capture what exists so far as well as I understand it:
There are two types of custodian, NBT and NSR. Custodians are addresses that are awarded a Unit (NSR or NBT) because NSR holders pass a motion which includes it. Giving a custodian funds actually takes a separate vote, but a motion vote without the subsequent custodial vote would only hurt the share holders. The motion specifies the purpose of these units and their intention.
There is also a burn custodian, OP_Return, which makes the coins unspendable and effectively burns them.
Custodial humans, owners of custodial addresses, are fundamentally people paid by Nu to accomplish some goal. Of course, the main purpose is to maintain the peg, but really any job can be contracted through this system. A marketing custodian, for example, is a possibility. However, given the business model of Nu, there is a clear overarching rule:
A custodian provides some mechanism whereby the peg can be fixed and in this way pays for the units it is given.
With recent votes, decentralized custodianship is clearly a preferred route. What’s more, being shown with the Nupool, we can clearly automate the process such that the process can be completely anonymous and unassociated; a user can prove to Nu that it held a wall up for just 1 minute and they will be credited.
My question comes down to this: what are custodians when Nu makes the market? A custodian gets paid for keeping a peg, such as btc/nbt, by announcing its buy or sell order to be within a number determined by other exchanges. If most traders start acting as Nu custodians because it’s easy, decentralized, and profitable then we need some mechanism to let volume push the custodians. Otherwise we are pegging btc along with nbt.