As far as filters indoor, I wouldn't use any as its not nearly as bright as outdoors. You normally use ND filters outside as you want to control the shutter speed on very bright days. If you're taking stills, it's not as necessary as shutter speed is irrelevant for cinematic blur; Unless you're intentionally using artistic blur in your stills. ND stands for neutral density. It transmits equal amounts of the RGB colors; thus simply reducing the amount of light hitting the sensor allowing for longer shutter speeds at a given light brightness. The ND4, ND8, ND16 etc mean that 1/4th, 1/8th 1/16th etc amount of light comes through. ND16 is a 4-stop reduction in light. That's a complete different discussion as to what that means in the photography world. It's a lot of drone this
Phantom 4 Pro! Cheers!