In DESelect you can define what we call a Custom Value, which is a value that you're going to add to your results that is not coming directly from your source data.
Custom Values are available on the Target Definition screen if this functionality is enabled in your DESelect plan. You'll find it on the left-hand side on the screen under the Available Fields section.
The Formulas Custom Value type provides different functionalities that can help you manipulate existing date data, make calculations with them and generate new ones.
More details on the available functionalities and how they work can be found below: