In most cases the measurands or counters are located in single 16-bit registers. The measurands are either unsigned or signed two's complement values while the counters are always unsigned values.
In some cases the measurands or counter values can be located in two consecutive registers, thus forming a single 32-bit integer value. The 32-bit value is always coded so that the high word part, that is, the higher 16 bits, is located first in this register address. The low word part, that is, the lower 16 bits, is then always in the next register address.
Register sizes and types are clearly stated in the Modbus memory map list.