Gittin 9:2-3

Gittin 9:2

If a man divorces his wife specifying that she may marry any man except for his father, her father, his brother, her brother, a servant, a non-Jew and anyone whose marriage would not take legal effect, the get is valid. (Since these people are prohibited to her anyway, the “condition” adds nothing.) If he specifies that she may marry any man except for a widow to a Kohein Gadol, a divorcee or a woman who performed chalitzah to a regular kohein, a mamzeres or a Gibeonite woman to a regular Jew, the daughter of a regular Jew to a mamzer or a Gibeonite man, and anyone whose marriage would take effect, albeit impermissibly – the get is invalid (because it does not allow her to marry any man).

Gittin 9:3

The main idea of a get is that the woman is permitted to any man. Rabbi Yehuda says the get must state, “This is a bill of divorce, a letter of release and a document of dismissal from me to you so that you may go and marry any man that you want to marry.” The main idea of a document releasing servants is that the woman is free and she belongs only to herself. (The same applies to male servants, the mishna just happens to be speaking of women.)