Article 159 bis 13 outlines the rules for expense deductions for small enterprises. While following the general principles of Articles 54 to 64, the specific rules for enterprises are determined by the Executive Regulations. Crucially, the enterprise can only deduct expenses and costs that were actually spent to realize gross income, provided they are accurately recorded in the entity's books and supported by documentation. This ensures that even under a simplified regime, only legitimate business expenditures are allowed to reduce the taxable base for the 3% enterprise tax.