IOB2
Another similar format which is widely used is IOB2 format, which is the same as the IOB format except that the B- tag is used in the beginning of every chunk (i.e. all chunks start with the B- tag).
IOBのWikipedia（英語）より（例もあってわかりやすい） IOB2はラベルは絶対Bで始まる（Iで始まることはない）
IOBはラベルがBで始まるとは言えない