Well, open caps are usually red. An open cap would be like what Niall said. And also, they are used to preserve the receptacle from weathering. All it is is an empty red photocell housing. I got mine because the power company shut off the light in front of my house because the city wanted to save money. After tons of complaints, the lights got turned back on and I got to keep the open cap. Nothing really special about it. It does the same thing that nothing would do exept the open cap tells the utility that the light is suposed to be off.