Headings are the basic set up for any given scene. This means that at the beginning of a filmic scene on your screenplay the heading will set the stage for a few different elements so that the events of the scene can unfold within the set parameters. Like all aspects of the screenplay process there is a generally accepted format for these headings. Master scenes, which are the main theatrical scenes requiring usually more than one camera perspective, have a unique pattern for their headings all their own.
You can line up the heading for a master scene in two or three separate segments. For all parts of this heading you should be putting all text in caps as to separate it from the dialogue parts of the script. The first segment of the heading concerns where you are going to be filming this. For example, this can mean whether or not this is going to be in an external location or an internal one. EXT. stands for external location and INT. stands for internal location.
The next section will be where the location is at. Is it in an open park or a backyard? Is this a home office or a cubicle in a large business community? List quickly where you are at in a general sense. Do not give too much detail as this will slow down the reading process and take away control from the director, yet do not be too vague otherwise the reader will not have a clear idea of the significance of the location.
The last part, which some people ignore, is the time of day the scene is taking place. This can be the morning, noon, afternoon, evening, late night, or really as specific as you want to be. Even though this is not always included you may want to go for it anyway to communicate more to the reader and maintain the mood and progression of time.
You may have to use more than one master heading in one scene if you move from an interior to an exterior or visa versa. If another scene occurs in the same location you can just write something like SAME or TIME PASSED to indicate that instead of writing an the same master heading over again. This can help stave off confusion that may happen with two different scenes sharing the same master heading.