How does the Tax page decide which companies can share group relief?

Tax → Group reads Company Structure as at the year end and checks it against the Companies House PSC register to say which companies form a proven 75% group; only a proven group may surrender and claim, and the split you enter must balance. A company that can do neither (e.g. a foreign subsidiary) is named and left out. With one company the tab says 'One company — no group to form'; with no shareholdings it says 'Company Structure is not filled in'. It is an estimate for planning, never a group relief claim.