They are particularly helpful when house training or for limiting a pet to a small area when you leave the house. Metal wire-type crates are not airline approved. Placing a crate pad inside will make it more cozy for your pet. These are typically used for cats, but they may be used for transporting very young puppies a short distance, for example, to the veterinarian. They are large, they may or may not have a top, and are constructed of metal.
Some pens are collapsible so that they can be easily moved or stored.
They are installed in the opening of a doorway, but still allow you to pass through. Consider a dog door. Some doors are installed in an existing wall or door. Other doors are installed as a separate panel that serves as an extension to a sliding glass door. In all cases, pet doors can be locked to keep the pet inside or outside, and to be an added security.