Difference between indicators on real time vs historical data

[pkatta]

Hello,
I have been trying to out the differences between indicator code that works only for realtime OR code that works on historical data only OR code that works on both realtime and historical data.

Why do some indicators work only on realtime data? What prevents an indicator from working on historical data?

In terms of the indicator code getting kicked off, what is the difference? Are different callback methods invoked in the indicator code for historical data vs real time data?

Thanks,
PK

[eDanny]

Some information like the ticks in bars, bid/ask, market depth data is not available in most historical data. If the indicator is using that data it won't find it in the historical data and therefore can't plot anything.