An introduction to Daml -> 4. Transforming data using choices

Hey, in the code below, Alice transfers money to Bob. It’s possible because Alice is an owner , and the owner is the controller of Transfer.
During Transfer choice, after the “do” block, its create a new SimpleIou with the new owner(in our case is Bob).
Can you explain how it’s possible?
I thought that only the signatory of SimpleIou(in our case “Dora”) can create a new SimpleIou, and in this case the creation happen during the Transfer choice, which Alice implemented.

module SimpleIou where

import Daml.Script

data Cash = Cash with
  currency : Text
  amount : Decimal
    deriving (Eq, Show)

template SimpleIou
    issuer : Party
    owner : Party
    cash : Cash
    signatory issuer

    controller owner can
        : ContractId SimpleIou
          newOwner : Party
          create this with owner = newOwner

test_iou = script do
  alice <- allocateParty "Alice"
  bob <- allocateParty "Bob"
  charlie <- allocateParty "Charlie"
  dora <- allocateParty "Dora"

  -- Dora issues an Iou for $100 to Alice.
  iou <- submit dora do
    createCmd SimpleIou with
      issuer = dora
      owner = alice
      cash = Cash with
        amount = 100.0
        currency = "USD"

  **-- Alice transfers it to Bob.**
**  iou2 <- submit alice do**
**    exerciseCmd iou Transfer with**
**      newOwner = bob**

  -- Bob transfers it to Charlie.
  submit bob do
    exerciseCmd iou2 Transfer with
      newOwner = charlie

In order to create a SimpleIou contract, you need to have the authority of the issuer, not necessarily be the issuer.

In this case, here’s how to read this. The issuer (dora) very deliberately delegates to the owner (alice) the ability to create a new SimpleIou provided that the currency, amount and issuer are the same and the existing SimpleIou is destroyed in the process.

Another way to think about it is that, within the code of the Transfer choice, you have the full authority of both the controller (alice) and all of the signatory's of the contract.