Hi Nick,
Workfront will go through your IF statement sequentially and will stop when a condition is met. So following your example:
IF(Field A<=99999,1,IF(Field A<=299999,3,IF(Field A>299999,5,””)))
If Field A had a value of 500, it will return true on the first condition (<=99999) and output '1'. It would not go onto the the next condition and return '3' as well (even though 500 is less than 299999) because the first condition had already been met. For this reason, you wouldn't need to write a statement that looks between two values, instead a simple less than or equal to condition will suffice for the middle portion.
However, if you did have a scenario where you needed to fulfil two conditions as part of the same expression you could use the '&&' Operator. Continuing with your example, the below statement would deliver the same outcome as what I have described above.
IF(Field A<=99999,1,IF(Field A>99999 && Field A<=299999,3,IF(Field A>299999,5,””)))
Hope that helps!
Best Regards,
Rich.