标题：请问语句含义

HighD:=callstock(stklabel,vthigh,6,-1);
LowD:=callstock(stklabel,vtlow,6,-1);
CloseD:=callstock(stklabel,vtclose,6,-1);
为什么这个 表示 求昨天的价格用

If (HOLDING=0 ) THEN BEGIN
If (High>=UpperBand) THEN
Buy(HOLDING=0,lots,LIMITR,Max(Open,UpperBand));
If (Low<=LowerBand) THEN
BuyShort(HOLDING=0,lots,LIMITR,Min(Open,LowerBand));
END

If (HOLDING<0) THEN BEGIN
If (High>=UpperBand) THEN BEGIN
SELLSHORT(HOLDING<0,lots,LIMITR,Max(Open,UpperBand));
Buy(HOLDING=0,lots,LIMITR,Max(Open,UpperBand));
END
END

If (HOLDING>0) THEN BEGIN
If (Low<=LowerBand) THEN BEGIN
Sell(holding>0,lots,limitr,Min(Open,LowerBand));
BuyShort(holding=0,lots,limitr,Min(Open,LowerBand));
END
END
还有这3组，有什么不同。