The per diem is as set internally by the organisation and is its standard rate (usually no higher than the amount that the State tax authorities allow). It should be clear what it covers (e.g. meals, entertaining, drinks, taxis(?) etc).
Hotel costs (room etc) must be evidenced by invoice (don’t forget “foreign” VAT on these invoices is also not allowed). Preferable if taxis, buses, phones, internet etc also evidenced by receipt/invoice (all net of VAT that can be identified). Normally per diem will therefore only cover meals and soft drinks/coffee, so should not be a very high figure.