What is declare upper in ThinkorSwim?

declare upper;

#
# TD Ameritrade IP Company, Inc. (c) 2009-2020
#

declare upper;

input trailType = {default modified, unmodified};
input ATRPeriod = 5;
input ATRFactor = 3.5;
input firstTrade = {default long, short};
input averageType = AverageType.WILDERS;

Assert(ATRFactor > 0, "'atr factor' must be positive: " + ATRFactor);

def HiLo = Min(high - low, 1.5 * Average(high - low, ATRPeriod));
def HRef = if low <= high[1]
    then high - close[1]
    else (high - close[1]) - 0.5 * (low - high[1]);
def LRef = if high >= low[1]
    then close[1] - low
    else (close[1] - low) - 0.5 * (low[1] - high);

def trueRange;
switch (trailType) {
case modified:
    trueRange = Max(HiLo, Max(HRef, LRef));
case unmodified:
    trueRange = TrueRange(high, close, low);
}
def loss = ATRFactor * MovingAverage(averageType, trueRange, ATRPeriod);

def state = {default init, long, short};
def trail;
switch (state[1]) {
case init:
    if (!IsNaN(loss)) {
        switch (firstTrade) {
        case long:
            state = state.long;
            trail =  close - loss;
        case short:
            state = state.short;
            trail = close + loss;
        }
    } else {
        state = state.init;
        trail = Double.NaN;
    }
case long:
    if (close > trail[1]) {
        state = state.long;
        trail = Max(trail[1], close - loss);
    } else {
        state = state.short;
        trail = close + loss;
    }
case short:
    if (close < trail[1]) {
        state = state.short;
        trail = Min(trail[1], close + loss);
    } else {
        state = state.long;
        trail =  close - loss;
    }
}

def BuySignal = Crosses(state == state.long, 0, CrossingDirection.ABOVE);
def SellSignal = Crosses(state == state.short, 0, CrossingDirection.ABOVE);

plot TrailingStop = trail;

TrailingStop.SetPaintingStrategy(PaintingStrategy.POINTS);
TrailingStop.DefineColor("Buy", GetColor(0));
TrailingStop.DefineColor("Sell", GetColor(1));
TrailingStop.AssignValueColor(if state == state.long
    then TrailingStop.Color("Sell")
    else TrailingStop.Color("Buy"));