It is the last populated field required which validates that all of the required FD and VD have been completed correctly in order to move from one critical task to another.


For example, suppose in the Home Selling example that a customer contacted the Realtor in order to sell the home. The fixed data would include the property address, the homes total square footage, etc. In other words,  the information that is fixed. 


The variable data would include the sales price, the appraised value, the available financing, etc., the information that could change. For example, when moving from one Critical Task to another, like the negotiated purchase price. Although we know the customer wants to buy the home and the seller wants to sell the home, we must have some form of communication recorded before we can move on to the next critical task; which is receiving a signed contract.


For this to happen, we would require a signature from the seller agreeing to the terms of the deal. Proof this has been accomplished would be the critical field. It would be the last field populated, which would verify all of the other fields required to move on to the next step