The IF function itself can't directly be used within Data Validation to restrict user input in Excel. However, you can leverage it in conjunction with Data Validation's custom formula option to achieve data validation based on specific criteria.

Here's how it works:

1. Define your validation criteria: Determine the conditions for valid user input. For example, maybe you only want numbers between 1 and 100 entered into a cell.

2. Build the IF formula: Construct an IF formula that evaluates the user input. In this case, it would be something like: `=A1>=1 AND A1<=100` (assuming data entry occurs in cell A1). This formula checks if the value in A1 is greater than or equal to 1 and less than or equal to 100.

3. Set up Data Validation: Select the cell(s) where you want to enforce validation. Go to the "Data" tab and click "Data Validation" in the "Data Tools" group.

4. Use Custom Formula: In the "Settings" tab, under "Allow," choose "Custom formula" from the dropdown menu.

5. Enter the IF formula: In the "Formula" box, paste your IF formula (e.g., `=A1>=1 AND A1<=100`).

6. Error Message (Optional): In the "Error Alert" tab, you can create a custom message that appears if the user enters invalid data. This message can explain what kind of input is expected.

7. Click OK: Click "OK" to close the Data Validation window.

Explanation:

When a user enters data into the cell, Excel will automatically evaluate the IF formula within the Data Validation. If the formula returns TRUE (meaning the input meets the criteria), the data is accepted. However, if the formula returns FALSE (meaning the input is invalid), Excel will trigger the error message you defined (if any), preventing the invalid data from being entered.

Note: This is a basic example. You can use more complex IF statements with various conditions to achieve more intricate validation rules.