<h1>ITR-1 validation rules require return software to enforce data-quality checks and categorize defects affecting uploadability.</h1> Validation rules for ITR-1 classify defects into three categories with specific operational effects: Category A defects prevent upload (e.g., income computation mismatches, tax payment inconsistencies, PAN/name mismatches, excessive house property loss, Schedule 80G PAN reuse), Category B defects allow upload but trigger notices for unpaid tax, and Category C defects flag schedule detail omissions or salary variances prompting alerts to taxpayers and software providers.