Into numeric fields in settings windows you usually simply type a number. However, you can use any numeric expression instead.
For instance, suppose you want to create a Sound that contains exactly 10000 samples. If the sampling frequency is 44100 Hz, the duration will be 10000/44100 seconds. You can create such a Sound by choosing Create Sound from formula... from the New menu, then typing
into the End time field.
Into text fields in settings windows, you can only type text directly; there is no way to use string expressions (except if you use scripts; see Formulas 1.8. Formulas in scripts).
Into numeric vector fields in settings windows, you can type any numeric vector (array) expression.
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