Corporate plans are plans created by insurers that are marketed to large organisations. You are entitled to these plans regardless of whether or not you are a member of the group to whom it is being marketed. In some cases, these plans may offer better value than those plans marketed towards individuals. An insurer cannot refuse to sell you any plan you request.
Corporate plans are sometimes confused with the group discount, which is a discretionary discount of up to 10%, which an insurer may give to a group of persons.