Block from U+31A0 to U+31BF. This block was introduced in Unicode version 3.0 (1999). It contains 27 codepoints.
The Wikipedia provides the following information on block Bopomofo Extended:
Bopomofo Extended is a Unicode block containing additional Bopomofo characters for writing phonetic Minnan, Hakka, Hmu, and Ge languages of China. The basic set of Bopomofo characters can be found in the Bopomofo block.