Input variable is the last functionality under New Variables/Values section. If you are creating a Data field in the report type called Form than you will need to fill the data on the respondent level. The new variable that you can use in the Form is named “Input Variable”. The input variable is similar to a Computed variable in setup but some differences.  

Input variables can now be created and edited from the new Questions page with the same logic as computes, merged, and index variables are managed on the same page.

The Input variable option is shown last in the Create variable panel, see below.