The sunspec specification uses “pages” or “block” to collect report data. Each device is free to decide what to implement. There is a discovery method built into the specification, so one can start from the beginning, read and identify all the blocks exported, and skip those that are not understood by the reader.
Our sunspec implementation in OH does this too. If you turn on auto discovery on the modbus bridge, and your device exports any of the known blocks, then they will show up in the auto discovery as things that you can use.
If however the meter type does not show up, it is very likely that it is not supported by your device, or uses a proprietary/newer block type than what is implemented at the moment.
If you turn on logging for the modbus package you could see the autodiscovery iterating through the block types found, this could be an indication of what is happening.
The manual setup is there for a last resort. Use this only, if you’re pretty sure that there is a block that matches the one we’re looking for, but for whatever reason autodiscovery did not found it. But it’s really a “hackers tool”, and not for everyday use.