It is used in legal proceedings to indicate that a party is required to substantiate or prove their claims or assertions with adequate evidence. The defendant is being required to provide proof concerning the standard hire rate of a car from a mainstream or reputable local supplier.

Relevant Example:
In a typical legal scenario involving contractual disputes or claims regarding services or goods, a party (like the defendant in your example) might be put to proof to demonstrate that the terms they adhered to were standard or agreed upon. For example, in a case where there is a dispute over the cost of hired services (such as car rental rates), the court might require the defendant to present evidence supporting their claims about the prevailing market rates.

Working References:
Judicial Commission of New South Wales - Sentencing Trends No. 17:
This document discusses various legal principles and includes references to cases where parties are put to proof, particularly in relation to evidential burdens in sentencing and legal disputes.
https://www.judcom.nsw.gov.au/sentencing-trends-17/

Special Court for Sierra Leone - Decisions:
This document details how parties in criminal proceedings are put to proof, illustrating how the prosecution or defense is required to substantiate their claims through evidence.
https://www.rscsl.org/Documents/Decisions/Taylor/171/SCSL-03...