What is a letter of approval?

In general, written approval from a Party involved (called the letter of approval) is the authorisation from that Party of the participation by the project participant in the proposed project activity:

A written approval constitutes the authorization by a designated national authority (DNA) of specific entity(ies') participation as project proponents in the specific CDM project activity (EB 16, Annex 6, paragraph 2(c)).

A host Party's letter of approval for a PoA will cover all CPAs included under that PoA which are located within the territory of the party. It is not necessary to obtain a separate letter of approval for each CPA.