Under Article 25, a submitted tax declaration is deemed the official assessment unless amended by the Tax Department, which can also define income and tax on an estimation basis. The Department is authorised to issue a re-assessment if it discovers unassessed income or identifies a mistake in the original assessment, provided it is within the statute of limitations for the tax debt. The taxpayer must be notified of any such amended assessment. The law's executive regulation is designated to define the specific terms and procedures for enforcing the provisions outlined in this Article, ensuring procedural clarity for all parties.