https://www.fincen.gov/sites/default/files/shared/FIN-2013-G001.pdf
The definition of a " Money Transmitting Business " is a subset of a Money Services Business which Wikipedia defines as
An MSB has specific meanings in different jurisdictions, but generally includes any business that transmits money or representatives of money, provides foreign currency exchange such as Bureau de changes, or cashes cheques or other money related instruments. It is often used in the context of Anti Money Laundering (AML) legislation and rules.[1][2]
The beauty ( and bane) of cryptocurrency is that the IRS and several other cases have ruled crypto as Property and not money. I'm no lawyer but if its consider property then nodes are not considered money transmitting businesses and do not need FINCEN registration. If you are concerned, it would be best to check with a business lawyer preferably one that deals with cryptocurrency.
Regarding Mining/Staking and similar:
I can't speak for everywhere but the IRS guidance for cryptocurrency mining is that it is considered income.
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"A user who obtains convertible virtual currency and uses it to purchase real or virtual goods or services is not an MSB under FinCEN’s regulations"
"an administrator or exchanger is an MSB under FinCEN’s regulations, specifically, a money transmitter, unless a limitation to or exemption from the definition applies to the person."
"An administrator is a person engaged as a business in issuing (putting into circulation) a virtual currency, and who has the authority to redeem (to withdraw from circulation) such virtual currency"
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I am not a lawyer(!) but my understanding of this is that a notary node (or anybody that is mining) is putting into circulation a virtual currency but there is no redeem powers as the KMD that is mined cannot be destroyed. Clever lawyers might try to argue that anybody can burn by sending to a null pubkey, but it is not actually destroyed, just very unlikely to be able to be spent. Also, any such "authority" would be something all users would have and as such is not fitting the context of an administrator. After all, how can everybody be an administrator?
It would be nice if someone with legal experience in this can confirm that mining crypto does not make an administrator. It seems to apply more to gateways.
In more good news the DEX I think fits into the end user category as each DEX swap is an end user paying crypto in exchange for crypto.
From what I can tell, it is the providing of services to third parties that is what is being regulated. Direct peer to peer commerce does not seem to be covered by this, but again this is not any legal opinion, just my reading of the PDF above
By administering they mean conversion and facilitation. A notary node is not facilitating conversion of value as a product or a transaction resulting in Fiat. The Notary Node provides a service to which payment is rendered and that is income. This is taxable. But as long as you don't convert into Fiat or Products or other Services there has been no Profit. If you trade KMD for a Car you now have to show a Profit as the Car's worth. If you traded KMD for BitCoin you still have not made a profit. If you buy a physical item with that BitCoin you now have a profit. If you cash it out to pay your electricity, you now have a write off for electricity of running a node and costs of ISP and equipment etc. and the rest is profit. I don't have to be a lawyer to know this, it is pretty obvious.