This is the flag agreed by a meeting of Chiefs in 1834 and adopted as official until 1840. (It is actually the flag of the Church Missionary Society, hence the crosses.) It is the flag used by the Chiefs who signed the 1835 Declaration of Independence. It is the flag Hone Heke wanted to fly beside the Union Jack at Kororareka (Russell), and why he repeatedly cut that flagpole down. It is increasingly being favoured as a flag of those who support the 1835 Declaration of Independence, which predates the Treaty of Waitangi, and it's considered less confrontational than the Tino Rangatiratanga flag.
The note with it is one that was sent (with a diagram) to the Governor of New South Wales (and New Zealand) who forwarded it to King William IV. The King approved the flag, and a drawing of it was circulated through the Admiralty with instructions to recognise it as New Zealand's flag. It came to be known as the Flag of the United Tribes of New Zealand .