Within the legal mobility budget, most of the pillar 2 expenses will be prefunded by the employee. Think about an employee buying a bike, paying the rent, ordering a taxi, ... When adding an expense, the employee should upload a justification document.
It is up to the employer to determine what justification must be submitted to prove the expense. No specific formalities are imposed.
In the event of social or fiscal control, it is up to the employer to provide proof, via the justification documents submitted by the employee. This needs to prove that the use of the funds in pillar 2 complies with the legal conditions (e.g. that the interest on a mortgage loan relates to the place of residence of the employee within a radius of 10 km - as the crow flies - from his/her usual place of work).
Examples of frequently used justification documents:
Contract in case of rental costs
Repayment table in case of the interest on mortgage
Check out ticket in case of the purchase of a vehicle