Convention definitions clarify applicant/requested State, tax and tax claim; domestic law fills gaps and annex changes notified. Article 3 defines key Convention terms: 'applicant State' and 'requested State' for administrative assistance; 'tax' to include taxes and social security contributions; 'tax claim' as unpaid tax amounts with interest, administrative fines and recovery costs; 'competent authority' as those listed in Annex B; and 'nationals' to include natural persons with the Party's nationality and entities constituted under its laws, with further clarifications in Annex C. Undefined terms are to be interpreted under the law of the applying Party for covered taxes, and Parties must notify Depositaries of Annex B and C changes, which take effect after a three month notification period.