A new construction housing is normally installed before the ceilings are in place.  It is made to be be held without the necessity for the ceiling. These recessed fixtures are easily installed between ceiling joists of new building or major remodeling projects.

If the ceiling is already in place, then you would select a remodelling housing.  The ceiling holds the fixture in place. These fixtures are designed for remodelling applications and can slip into an existing ceiling space through holes made to accommodate wiring.

Consult your electrician to see which housing is best for your needs.