Evaluate for a different stock/contract or strategy or bar sizse
item_reference - either a stock symbol or a strategy name
expression - formula
The default context in all formulas is the current stock for bar data items and the current strategy for daily stats items.
Extern allows you temporarily change context to a different stock or strategy.
To reference a stock, use a $ to prefix the symbol of the desired stock, e.g. Extern($MSFT, C)
To reference a strategy, use a @ to prefix the name of the desired strategy, e.g. Extern(@mr_short, S.Equity)
To calculate a formula using a specific bar size, use a ~ to prefix the name of the bar size, e.g. Extern(~Weekly, MA(C, 20))
Note that the expression referenced can be any formula, and it is most efficient to do as much as possible in a single Extern function. For example, Extern($SPY, C > MA(C,20)) would be preferable to Extern($SPY,C) > Extern($SPY, MA(C,20)) or, even worse, Extern($SPY,C) > MA(Extern($SPY,C), 20) -- though all would return the same result.
If a stock symbol already starts with a $, e.g. $SPX, you will have to add an other $, hence Extern($$SPX, C).
There are also special types of Extern references available for individual futures contracts and corresponding industry index symbols.