TL;DR
High throughput belts designed for moving items between factory segments but not within them.What ?
Belts capable of carrying either 135 or 225 items/s with an accompanying set of undergrounds (+2 length) but no splitters. These belts move too quickly for inserters to pick up or put down on. They will not connect directly with normal tiered belts for input or output.Instead of normal splitters they have loader/unloaders that must connect to express belts on one side and hyper belts on the other. The loader should have a 45 item buffer per input and will only output once each buffer is full. Likewise the unloader can only unload equally onto all outputs.
Why ?
There is a need for high throughput belts in megabases, but the developers (rightly) do not want to just add just a simple straight upgrade as it would add nothing to the gameplay other than needing another cycle of upgrading existing lines. The intention with this design is to provide high throughput at the places it really matters (high traffic stations, smelting etc to high production areas such as green circuits) but can't be used as a direct upgrade. Instead it takes some of the mechanics of belts that players are already used to but forces different approaches.The loader/unloader mechanic in particular requires players to have even throughput on both ends to avoid stuttering or backing up. As multiples and powers of 2 are common for scaling I went with it taking 3 or 5 belts in order to throw more complicated ratios. If it was 4 belts worth it would be easy to upgrade main supply lines but this requires a bit more thought and planning at each end.