is the standard functionality which provides the ability to access data that is within the system and then fetch the data to the appropriate budget Assumptions Sub-tab. Non-system data can also be imported in the Assumptions Sub-tab using the Import Budget Assumptions screen. Then using the Row-to-Row Calculations functionality, math functions can be applied to the accessed data.
Please see the Lookup and Row-to-Row Calculation articles for more details
Please also review these videos for more information: