A model or property release is a legal document signed by the subject, or the owner of the subject in the element. It grants you permission to feature the subject in the element, in one form or another. The subject can be in the form of people (model) or properties.
A release is required as long as the elements have any recognizable faces and private properties. Usually footage taken inside or prominently featuring private properties would require a property release.
The purpose of the release is to protect you from any future lawsuits the person might file for claims such as defamation and invasion of privacy and is required for commercial purposes such as advertisements or promotions.
A property can refer to buildings, structures, monuments, cars, boats or collectibles.
Commercial purposes generally mean that the element is used to promote a product, service, or an undertaking which is intended for profit. This includes use in advertising, marketing, promotion, packaging, publication covers, advertorials and consumer or merchandising products.
If you are unsure of whether a model or property release is required, please consult your legal advisor.