Does the scene take place indoors or outdoors?
Use abbreviations. No need to to type out INTERIOR or EXTERIOR. Just use INT. or EXT.
From Eternal Sunshine of the Spotless Mind by Charlie Kaufman:
INT. BOOKSTORE – NIGHT
Joel sits in the bookstore coffee shop. It’s a jarring transition, visually and emotionally. Joel is in the midst of some traumatic state of mind.
In the broad sense of the shot heading, you don’t need to give people everything. Just give ’em what they need to get into the scene. In the above example, Charlie uses Interior (INT.), Location (BOOKSTORE), and Time of Day (NIGHT).
What if my scene takes place both indoors and outdoors?
Well, there’s one for that, too.
From The Girl with the Dragon Tattoo by Steve Zaillian:
INT./EXT. COTTAGE – DAY
Blomkvist sifts through a box containing some of Harriet’s personal belongings: school papers, textbooks, a Bible, the address book, her wallet and ID ...
A knock on the door. He puts the box in a closet on top of others, kicks it closed, opens the front door to find a not unattractive woman in her 50’s on his porch.
Hi. I thought I’d come over and say hello. I’m Cecilia.
Now, Steve actually uses INT/EXT. his indoor / outdoor scenes, but the Scrippets plugin I use for these wonderful bits of screenplay text doesn’t recognize that format. The thing here to take is that Steve uses it with every instance of an indoor / outdoor shot. That being said, if you’re inclined to copy Mr. Zaillian, I would just make sure you’re consistent. Here are the two most recognized formats: