AccessibleText.SENTENCE is specified as

static final int SENTENCE
    Constant used to indicate that the part of the text that should be retrieved is a sentence.

Problems with understanding of SENTENSE arose while CR 4942444 discussion. So, this term seems to be not well specified.
Example: In AccessibleJLabel implementation string "Test. test2 Test3. test4." is treated to be the sentence. But it is not clear why.
As a result JCK test fails:
api/javax_swing/JLabel/AccessibleJLabel/index.html#AccessibleText[AccessibleJLabel0015]