Halakhah for Sanhedrin 37:22

The laws of the commandment: For example, the differences that the Sages, may their memory be blessed, revealed to us that there are in this commandment between one man and another, such that not every man is obligated to come in front of the court to testify to them: As if the witness was a great sage and the court less than he, he may - if he wants - withhold from testifying in front of them; as the positive commandment of honor of the Torah pushes off the positive commandment of testimony (as is elucidated in Shevuot 36b). And a high priest is also not obligated to testify except only for testimony [that pertains] to a king (Mishneh Torah, Laws of Testimony 1:3). And kings of Israel do not testify about others, and others [do not testify] about them, because of a case that happened, as it appears in Sanhedrin 19a in the chapter [entitled] Kohen Gadol. But kings of the House of David testify and [others] testify about them and judge them. And we do not withhold from accepting the testimony on account of love or hate, as 'the remnant of Israel will not do injustice' in their testimony (Sanhedrin 27b). But regarding judgement, it is not so; as a friend and an enemy may not judge, because the enemy cannot see a merit and the friend cannot see a liability.