The substitution of collateral can be done with one message: a substitution message contains both the collateral to be taken out from the collateral held and the proposed new collateral.
The substitution can also be done in two steps: a first message is sent with only the details of the collateral to be pulled out and a second message is sent afterwards with the proposed new collateral. This is done when the giver wants to substitute collateral but doesn't know what collateral he will use at the moment of sending the first message.
When the taker requests the substitution, he will usually send a message containing only the collateral to be pulled out. The giver will then send a substitution message with the proposed new collateral.
The exchange of information related to the trade itself as well as to the settlement of the collateral is done with the existing confirmation and settlement messages of the relevant categories (categories 2, 3, 5, 7, 9).