NEC PSC: What relevance are Employer expenses in CD1?

In NEC3 PSC contract data 1 there is space for the Employer to list/state expenses, but how will these ever come into play during the life of the contract? i.e. when will they be applied??

It is a mechanism which allows the Employer to state what expenses they wish the Consultant to be responsible for (that is to pay on behalf of the Employer) and also to define what will be reimbursed, such as stating the amount per mile for car mileage or to limit travel to standard class fares etc

But where does the contract say how the Employer expenses will be applied?
Where does it say they can limit travel to standard class, when it say this is about Employer expenses??

Expenses is an identified term so the amount due under clause 50.3 relates to those expenses expressly stated in Contract Data Part 1 and 2. They are not expenses incurred by the Employer but a category of expenses identified by the Employer.