As such, exercise physiology professionals often have the responsibility of conditioning a person to a higher level of fitness and/or health while, at the same time, being aware of safety issues (risk of injury, illness, environmental exposure, etc.) associated with a single session of exercise.
A voucher is an internal document used in a company's accounts payable department in order to collect and organize the necessary documentation and approvals before paying a vendor invoice.
Invoice is what one would bill for payment with. Voucher is an internal document or documents the Government uses as the evidence to pay a bill. You might try looking in your agencies finance regualtions, policies, handbooks etc. as I suspect 'voucher will be defined there somewhere.