ModSnmp Manual v3.09 - 6.3.3. Cache settings
When ModSnmp reads a value from the Modbus device, it saves it in a cache. If the value is requested again (by an SNMP manager or a Modbus client) it can be returned from the cache instead of having to be fetched from the Modbus device again.
If polling is selected, the lifetime should be large enough to avoid excessive requests being sent to the device.
Cache Writes. When a manager requests a value to be written to the device, ModSnmp will normally retain the value in the cache if the write is successful. The value should then be available immediately when the manager requests it, without having to read it from the device.
Some devices have registers where the value read from the register is not the same as the value written to it. Cache Writes should be deselected for such registers.