First, a separate storm sewer system includes ditches, curbs, gutters, storm sewers, and similar means of collecting or conveying runoff that do not connect with a wastewater collection system or treatment plant.
Second, to be a "municipal separate storm sewer system" (MS4), the system must be owned and operated by a public agency, for example:
a city or town
a municipal utility district, flood control district, or other special district
a state or federal agency.
These can include municipalities and local wastewater districts, state and federal departments of transportaion, universities, hospitals, military bases, and federal correctional facilities.
MS4s are categorized by the size of the population they serve.