Exposure class is the input that everything durable about a concrete member depends on, and it is chosen once, early, often by whoever is fastest with the spreadsheet.
IS 456 describes the classes by environment rather than by building type. Mild is concrete protected against weather or aggressive conditions, except briefly during construction. Moderate covers concrete sheltered from severe rain, concrete continuously under water, and concrete in contact with non-aggressive soil. Severe covers alternate wetting and drying, and exposure to sea-water spray or corrosive fumes. Very severe and extreme escalate from there to tidal zones and direct sea-water contact.
The consequence is mechanical. Moving from moderate to severe takes nominal cover from 30 mm to 45 mm, cuts the maximum water-cement ratio from 0.50 to 0.45, and raises minimum cement content from 300 to 320 kilograms per cubic metre. That is a different mix design and a different bar schedule, decided by one word in a specification.
Two habits help. Classify per member, not per project — a basement retaining wall and a fourth-floor internal slab in the same building are not in the same environment. And write the class into the drawing rather than leaving it in the calculation file, because the person bending bars on site will never open that file.