Optional. Allows you to look back in time.

Examples:
O(D,-1) is yesterday's Open

O(D) is today's open

O(D,0) is today's open

Optional. Allows you to specify a particular symbol for a function.  If you omit the symbol, the function is based on the symbol in the current row of a quote sheet or the current chart.

Example:
O(D,,YHOO) is today's open for Yahoo.  (Note that "[Ref}" is omitted in this example.)

Required when listed as an argument for a function. 

Valid Values: 1,5,10,15,30,60 (any number between 1 and 1000 representing a time frame in minutes) and:
    D = Daily
    W = Weekly
    M = Monthly
    Q = Quarterly

Required when listed as an argument for a function. 

Valid Values:
   Open
   High
   Low
   Close