ModSnmp Manual v3.09 - 6. Object Types
SNMP and Modbus are both concerned with reading and writing the values of variables, but there are differences in how they do this. SNMP refers to its variables as "objects", and identifies them using OIDs (Object Identifiers), whereas Modbus refers to its variables with names like "Holding Register", and identifies them using addresses. SNMP and Modbus also use different types for their variables.
The Object Types pages enable you to set up mappings from the OIDs and types used in SNMP messages to the addresses and types used in Modbus messages.
The SNMP manager that you are using will also need to know about any SNMP objects that you configure here. This is done using an MIB file.