Use customCellEnterPre to change the behavior when users click in cells, before default Planning logic runs. For example, when users click in cells, a message can indicate the acceptable range of values to enter. When users click in certain types of cells, such as accounts, a message can prompt users to enter supporting detail.
After customCellEnterPre runs, if it returns True, the default Planning logic runs. If it returns False, no additional logic follows the custom code. SampleValidateData.js includes an example that displays a message prompting users to enter supporting detail when clicking in certain cells.