Who is the DNO?
The Distribution Network Operator (DNO) runs the local power network. Across most of Greater Manchester that's Electricity North West. Any system that can send power into the network has to be registered with them.
G98: connect and notify
For smaller inverter systems of up to 16 amps per phase, the installer can connect the system and notify the DNO within 28 days of commissioning. Most small home systems fall into this category.
G99: apply first
Bigger systems, including many homes with larger inverters or batteries and almost all commercial installations, need an application approved before connection. The DNO checks the local network can cope. This can take several weeks, and sometimes the DNO will limit how much you can export.
Export limitation
If the network can't take your full export, the system can be set up to cap what it sends to the grid. You can still generate and use (or store) everything on site.