Travel need assessment

Good morning,

I will be conducting intervieww with my Internal Customer to define their needs regarding travel.
There will be several chapters in the interview:
- Tariffs (is there any contract, statistics, preferred Vendor, travel policy...)
- Agency (is there a travel agency, a tool in place, cost per transaction)
- Payment and audit (how are expenses paid and checked)

According to youm, what are the key questions not to forget during the interview?

Thank you,

Estelle