ADDRESSING in READHOLDING REGISTER

I am using ModbusMaster.h library. I want to know what address do I have to put in the code to read a register with address eg: 40151.

I am reading the meter with register map as: https://www.elmeasure.com/storage/app/media/resources/documents/elmeasure-multifunction-meter-modbus-register.pdf

What address should I use in the below code:

result = node.readHoldingRegisters(0x4101, 16);

Why not that address directly?

result = node.readHoldingRegisters(40151, 1);