It checks direction of an array index.
[in] Checked array.
Returns true, if the specified array has the AS_SERIES flag set, i.e. access to the array is performed back to front as in timeseries. A timeseries differs from a usual array in that the indexing of timeseries elements is performed from its end to beginning (from the newest data to old).
To check whether an array belongs to timeseries, use the ArrayIsSeries() function. Arrays of price data passed as input parameters into the OnCalculate() function do not obligatorily have the indexing direction the same as in timeseries. The necessary indexing direction can be set using the ArraySetAsSeries() function.