Nôm character for ngây

It writes ngây, the Vietnamese word for being naive or dazed.

Chữ Nôm is the historical Vietnamese writing system that adapted Chinese character forms to write Vietnamese words. Several characters are recorded for ngây; this is one spelling, not the only possible written form.

Why does Nom Na Tong use a private-use code for it?

Its older font record uses a font-specific private-use address.

The Unicode source list labels the character VN-F0864; its notes say VN entries can refer to original private-use codes in the open Nom Na Tong font. A private-use slot is a number assigned within one font or text system; other fonts may not map it to the same character. In new Unicode text, use the assigned character 𱒇.