Constants are special number values that can be used to make Formulas with Number values or Money values in your data. A simple example would be to multiple an invoice amount with a constant sales tax amount. Constants can be changed at any time and the results of the formula will be always stay in sync.
Each constant is unique to a particular view. You cannot automatically re-use a constant on a different view, although you can create the same constant again.
If you have already added a constant to a view, it will appear in the Formula menu as an option if you want to add another formula column.

The constant’s value can be changed at any time. Simply edit the value on the left, and press Enter. Any formula using the constant will be re-calculated immediately.
Click the delete button (the X) beside the constant value on the left to remove it from a view. Any formula columns that use the constant will also be removed from the view.