Senators' Official Personnel and Office Expense Account (SOPOEA): History and Usage [Updated November 5, 2020] [open pdf - 1MB]
From the Summary: "The Senators' Official Personnel and Office Expense Account (SOPOEA) is available to assist Senators in their official duties. The allowance is provided on a fiscal-year basis (i.e., October 1-September 30). Funding is provided in the annual legislative branch appropriations bills. [...] This report provides a history of the SOPOEA and overview of recent developments, including funding levels. It also analyzes actual SOPOEA spending patterns in recent years."
CRS Report for Congress, R44399
Congressional Research Service: https://crsreports.congress.gov/