From the Executive Summary: "One of the greatest challenges in the formulation of USSS's [U.S. Secret Service] budget is determining the amount of protective travel that the agency will be required to undertake in a future fiscal year. In addition to the highly variable and unpredictable schedules of USSS protectees, unplanned National Special Security Events and unforeseen spikes in international travel can have a significant impact on the agency's travel budget projections. [...] This report fulfills the requirement to detail 'expenditures of funds for the purpose of providing protection in accordance with each of the categories listed in 18 U.S.C. 3056(a) during the course of any travel.' USSS currently provides fulltime protection for 32 statutory protectees, including the President, Vice President, their immediate families, former Presidents, their spouses, and any minor children of a former President who are under 16 years of age. By statute, USSS also provides temporary protection to visiting heads of state and foreign governments. The number of these visits varies from year to year but is especially high during the opening period of the General Debate of the United Nations General Assembly each September."
U.S. Department of Homeland Security: https://www.dhs.gov/