A per diem is a fixed daily allowance for meals and incidentals, and understanding how it works turns a confusing policy into a simple plan. The rate is usually tied to the cost of the city you are visiting, which is why it changes from trip to trip.
Plan against the rate rather than reacting to it. Knowing your daily allowance in advance lets you choose where to spend and where to save without tracking every receipt in a panic. In many policies, the per diem is yours to manage, so a frugal day gives you room on a client dinner.
Know the boundaries. Some programs still require receipts above a threshold or exclude certain items, so read the policy once and travel confidently after that.