A more thorough explanation:

sacramentalis (sak-rə-men-tay-lis), n. pl. [Law Latin from Latin sacramentum “an oath-taker”] is a historical term used to describe a compurgator. A compurgator is someone who takes an oath swearing to a defendant's innocence. The plural form of sacramentalis is sacramentales (sak-rə-men-tay-leez).

In medieval times, when someone was accused of a crime, they could ask their friends or family members to act as sacramentales and swear an oath to prove their innocence. For example, if someone was accused of stealing a cow, their brother and two neighbors might swear an oath that the accused was with them at the time of the theft and could not have committed the crime.

Another example of sacramentales can be found in the trial of Joan of Arc. During her trial, Joan was asked to swear an oath that she would tell the truth. Her response was, "If you were well informed about me, you would wish me to be out of your hands; for I have done nothing except by revelation." In this case, Joan was not acting as a sacramentalis, but the concept of taking an oath to prove one's innocence was still present in the trial.

These examples illustrate how sacramentales were used in the past to provide evidence of a defendant's innocence. By swearing an oath, the sacramentalis was vouching for the defendant's character and providing a form of testimony that could be used in court.