What restriction applies to worksheet filters in relation to user modifications?

The correct answer highlights that worksheet filters cannot include workbook or profile variables. This is significant because workbook variables are intended to apply broader changes across the entire workbook context, while profile variables relate to user-specific settings or adjustments. By preventing these variables from being used in filters, the system maintains a clear separation between general worksheet data and user-specific or workbook-wide settings, ensuring that filter functionality remains focused on the specific set of data defined within the worksheet itself.

This restriction ensures consistency and clarity in how filtering operates, allowing users to make effective and relevant selections based on their immediate data without the complexities introduced by variables that could alter the data context significantly.