High
Very little of what you feel reaches your face. The emotion still runs — suppression works on the expression, not the feeling — but from outside you look level, and in a crisis, a negotiation, or in front of a frightened child that composure becomes a skill other people lean on. Two honest costs travel with it. The feeling does not leave when the expression does, so it tends to stay with you longer than it would if it were voiced. And the people closest to you are working with very little: they are not reading you as calm, they are guessing. Letting one or two of them see the unedited version costs less than it seems.
Moderate
You show some of what you feel and hold back the rest, mostly depending on where you are and who is in the room. Close company gets more; a meeting or a difficult conversation gets the edited version. This is the most common arrangement and a workable one — you are neither performing composure at all times nor broadcasting every state you pass through. Worth noticing is that the choice is often not made consciously: certain rooms, certain people and certain topics bring the shutter down on their own. Learning which ones do that to you is most of what there is to take from this scale.
Low
What you feel is generally visible. You spend little effort keeping a straight face, and people around you rarely have to guess where you stand — which makes you easy to read, easy to trust and unusually easy to be close to. Emotion that gets expressed also tends to finish its course instead of sitting unresolved. This openness is not free: some rooms reward a flat face, and there are moments — a negotiation, a crisis, someone who needs you steady — when visible feeling makes things harder for you or for them. Being able to hold it briefly, by choice, is a useful thing to have available.