Explanation
Data validation rules are triggered when a user adds or changes a cell value.
Each time a number is entered, the validation is triggered.
As long as the sum remains less than 1000, validation succeeds.
If any entry causes the sum C6:C9 to exceed 1000, validation fails.
These values can be numbers, cell references, ranges, arrays, and constants, in any combination.
SUM can handle up to 255 individual arguments.
