The MODBUS data model is specified as having up to four tables (Coils, Contacts, Analogue Inputs, and Holding registers) that each contain elements numbered from 1 to n, corresponding to the internal (within the PDU of a MODBUS message) addresses of 0 to n-1.
When applying this specification to a product, the holding registers are traditionally mapped to numbers from 40001 through 49999 (offset from the data model by 40000), in order to avoid any confusion with other tables, which traditionally use other offsets. The offset used depends on the element type that a MODBUS function is intended to deal with, and always results in mapping the first element of the table to an internal address of 0.
The AquaVent only supports holding registers, and consequently only MODBUS functions that deal specifically with holding registers. Since register 40001 represents element number 1 in the holding register table of the MODBUS data model, it is given the internal address of 0. See Section 4.4 for the full AquaVent Register Map.