This is the particle fluid solver's upper limit of time steps.
The final used number of steps will be between both values.
In many simulations it is required to work with higher “FPS Output”.
With higher settings, e,g. 100 FPS, you will most probably receive an error message,
telling you that your “Choice of MAX substeps is too large”.
Fortunately, the error message tells you the maximum substep you should use for the current settings directly,
but you can also ignore this tip and start your simulation,
though this may result in longer simulation times.