It's a solar PV social energy cooperative bank in effect, whereby those with surplus solar PV generation in summer can donate this to support those facing fuel poverty and struggling with their energy bills in winter. Octopus SEG rate of 15p/kWh is donated in kind to one of two organisations, Octopus' own 'Octo-assist fund' (for existing Octopus customers) https://octopus.energy/blog/octo-assist/ or 'National Energy Action' https://www.nea.org.uk/.

I've chosen to support NEA as a registered charity with reach to non-Octopus customers, many of whom may be on pre-payment meters. My surplus solar PV is modest, around £100-150 each year. Most of our generation goes into the BESS, with any surplus offsetting the majority of the standing charge. But I would much rather donate this to those having to choose between eating or heating and genuinely fearful of their energy costs.

Solar Sharing came into fruition through the clever thinking of an Octopus customer. I've not heard of any other utility companies offering similar schemes.