Some indicators can be calculated on symbols which are different from the currently active symbol in your workspace (at the moment, all the lower indicators can do that). Every indicator like that has a special input parameter named "Symbol". In case if "Symbol" parameter is empty (it displays a gray "Current" label in this case), the indicator is going to work as usual, using the current symbol of the chart.
In case if you type in another ticker into the "Symbol" input, the indicator is going to start using this ticker to calculate. It's still going to keep using the primary time frame of the chart though.
Example 1:
Your chart is AAPL, Daily
You have an indicator, RSI(14), "Symbol" being empty (aka "Current")
In this case, you're going to have this RSI calculated using AAPL,D candles. If you switch the chart to another symbol (i.e., BA,60) then your RSI will switch to BA,60 as well.
Example 2:
Your chart is AAPL, Daily
You have an indicator, RSI(14, SPY), "Symbol" being "SPY"
In this case, you're going to have this RSI calculated using SPY,D candles. If you switch the chart to another symbol (i.e., BA,60) then your RSI will switch to SPY,60. RSI will keep using SPY until you clear the "Input" parameter or remove the whole indicator.
Currently, we don't limit the overall number of indicators using another tickers on your charts. The maximal number of symbols being used at the same time is 3 (including the main symbol), across all the plans.