What if has three options available to perform sensitivity analyses.

Spider Analysis Option
Use this option to analyze how the selected Output varies as each input does.
Input changes are expressed as a percentage of the initial value.

Tornado Analysis Option.
Use this option to analyze how the selected Output varies as each selected input does, with input ranges depicted as horizontal bars.

Sensitivity Tables Option.
Use this option to do "What if" analysis.
Analyze how one or multiple Outputs in your model vary as selected inputs take different values.
You can work with up to 20 Outputs and 10 inputs.

Note: An Input is any variable in your model which is not the result of a formula, but rather is part of one, and is not a string.
An Output is any variable in your model which is the result of a formula with numerical inputs.