Quantstrat signals and indicators

In the pairtrading demo in quantstrat R package the signals are added like this: 

pairStrat <- add.signal(strategy = pairStrat, name = "sigCrossover", arguments= list(columns=c("Ratio","BBands.up"), relationship="lt"), label="cross.up") 

This line above means the Ratio is crossing down below the upper Bollinger band. This means that the first provided column is less than the second after the crossing. 

The indicators are usually from the TTR package and can be looked up there for more info on how to use them.