Question: What is the latest procedure of BS IV (Euro IV) petrol-engined cars in PUC Centres?

As far as I know in Petrol Cars the car is kept idling without the Accelerator pressed, while in Diesel Cars the Accelerator is fully pressed.

But recently PUC Centres wanted to raise the engine speed for measuring petrol cars to 2500 rpm.

What is this aiming to measure and why would there be a change to to this new High Idle Emission Test procedure?

Answer: 
I can only give an answer for the part of the world where I live, which is Finland.

As default, in all cars with three-way catalyst, the vehicle is measured at idle and at >2000 RPM. Lambda value however is not measured at idle, only at >2000 RPM.

However, for Euro3 and Euro4 cars and newer, and also for cars sold after 1.7.2002, the idle check is replaced with an OBD check. The computer continuously monitors the emissions on the car, and thus, the amount of checks done manually is reduced. The measurements at >2000 RPM are still however done, and this includes lambda.

Lambda value not within 0.03 of 1.00 would be considered failed. However, if there's secondary air injection or lean burn, there may be exceptions.
**NB:** From the mention of PUC Centres I'm guessing you are located in India?

"Lambda" is, in simple terms a measure of how efficiently the engine is burning based on the air/fuel ratio in the exhaust gases emitted by the engine. How it's calculated is a bit more complicated than is appropriate to cover in a post here but there's a good explanation in [this](https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwiRrtebqofYAhUQEVAKHSj5DEIQFgguMAE&url=http%3A%2F%2Fcpcb.nic.in%2Fdivisionsofheadoffice%2Fpci3%2FLambda_report_No_14.doc&usg=AOvVaw2miT2-bGY5dQxjSDPXmPmt) document which also covers some of the motivations behind the tests being introduced in India.

As to why they would implement a high-idle test - I can't speak to their exact motivations but picking a standardized RPM value would eliminate the natural variance you see in "true idle" RPMs due to differences in cars and temperature conditions and make comparisons more effective. Additionally high-idle (or "loaded") emissions tests provide more useful information for checking for levels of NOx emissions, which given their public health implications are something of a hot topic at the moment.