Why does the Torah call the Choshen "Choshen Mishpat"?
Rashi #1: Because it atones for perversion of justice.
Rashbam: Because it informs the Mishpat of Yisrael (in connection with future events that require clarification).
Seforno (Pasuk 30): Because, the Kohen Gadol wore it on his heart in order to Daven on behalf of Yisrael that they should merit in judgment (before Hashem). 3
Targum Yonasan: It served the triple purpose of 1. conveying to Yisrael matters that were hidden from the Dayanim; 2. revealing to them war tactics; 3. atoning for perversion of justice.
According to the connotation of Mishpat that means to finalize the judgment - as opposed to the litigants' claims and the implementation of the judgment (Rashi). Rashi adds in Pasuk 30, that it was called Choshen Mishpat on account of the Urim ve'Tumim.