This feature is useful for highlighting cells that fulfill user defined criteria.
For example, if we have a list of numbers between 0 and 100 and we wish to see at a glance the numbers greater than 70, we can use this feature.

To do this, select the range of cells and choose Format -> Conditional Formatting. Key in the criteria and the format for the cells meeting those criteria as shown in the screen shots below.

The cells that contain numbers greater than 70 are now highlighted in yellow.

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This feature is used in the Linear Equation Solver to highlight the cells requiring data entry and in the Digital Rain tool to create the effect of the falling numbers.