The Securistyle Defender Friction Stay is a device that is used to hold open a window or door. It works by using friction to maintain the position of the opening. The stay is typically installed on the side of the window or door, and is designed to be adjustable so that the amount of friction can be adjusted to suit the weight and size of the opening. The size of the stay, in this case 300mm, refers to the length of the stay, and the model number A070 may be used to identify the specific type of stay being used. Coastal may refer to the location or environment in which the stay is intended to be used. It is important to follow the manufacturer's instructions for the proper use and maintenance of the stay, as well as any safety precautions that may be necessary when working with the equipment.