BERKELEY -- Alta Bates Summit Medical Center, a nonprofit corporation, is exempt from millions of dollars in property, income and sales taxes. In exchange, the hospital is obligated to provide benefits to the community. But at its July 16 meeting, the City Council expressed frustration at not knowing precisely what those benefits are. How much of the community benefits reported by the hospital goes to charity care, and how much of that is targeted to low-income Berkeley residents, council members asked. "Community benefits can be anything they decide," Dan Johnston, a researcher with the Institute for Health and Socio-Economic Policy, the research arm of the California Nurses Association, told to the council.