How-the-ATR-sizing-in-preferences-is-calculated

You're showing the use of the TASC ATRTrail indicator for % Risk Sizing. TASC indicators are documented by their articles in the magazine. Sadly our documentation doesn't even indicate which issue this one's from, so here's the code that populates the indicator.

CODE:
        public override void Populate()
        {
            BarHistory source = Parameters[0].AsBarHistory;
            Int32 period = Parameters[1].AsInt;
            Double factor = Parameters[2].AsDouble;
            DateTimes = source.DateTimes;

            //ATR
            ATR atr = new ATR(source, period);

            //calculate ATR Trailing Stop
            for (int n = period; n < source.Count; n++)
            {
                double loss = factor * atr[n];
                if (source.Close[n] > Values[n - 1] && source.Close[n - 1] > Values[n - 1])
                    Values[n] = Math.Max(Values[n - 1], source.Close[n] - loss);
                else if (source.Close[n] < Values[n - 1] && source.Close[n - 1] < Values[n - 1])
                    Values[n] = Math.Min(Values[n - 1], source.Close[n] + loss);
                else
                    Values[n] = source.Close[n] > Values[n - 1] ? source.Close[n] - loss : source.Close[n] + loss;
            }
        }

If you're looking for a straight ATR calculation for your Max Risk Sizing, you should do it yourself using the MathIndOpInd indicator.

p.s. ATRTrail was from the June 2009 issue of TASC magazine.
https://traders.com/Documentation/FEEDbk_docs/2009/06/Vervoort.html

QUOTE:
ATRTrail was from the June 2009 issue of TASC magazine.

If the ATRModified and ATRTrail indicators are both from the same TASC issue, then how are they different?

ATRModified is an inconsecuential modification (imho) of ATR.
ATRTrail is a value you can use for a SAR strategy.