Hi Pushkar, what you write is correct: the syntax with [f] turns out to be necessary only if you decide to apply any formula. This can be a comparison, through mathematical and logical operators (so "==", ">", "<", "!=", and so on), or should we want to apply any formula that Rulex allows to handle (simple examples can be the formulae "sum", "mean", "max", "min", and others).
Let me remind you that when using a formula, it is necessary to write the keyword [f] following the formula in question, separated by a space. The only cases in which you will not see [f] is when checking whether a column has a value that is part of a set, for which you then use the "in" operator (e.g., "Column A" in {'This value'}), in this case [f] is not required.
I hope I have made myself clear! Please, let me know if you have any other curiosities.
Thanks and have a nice day!