Structure determines who owns the economics. Governance determines who makes the decisions. They are not the same thing, and conflating them is one of the most common mistakes in private investment structures.
You can own 40% of an SPV and control it entirely — if the shareholders agreement gives you veto rights over material decisions, the right to appoint the majority of directors, and drag-along provisions that let you force a sale. You can also own 60% and be effectively locked out if a 30% investor has blocking rights over key matters. Percentage ownership tells you very little about actual control.
The levers of control in a private structure are: board composition and voting rights, reserved matters (decisions that require supermajority or unanimity), information rights (what data you can access and when), approval rights over transfers (who can sell to whom), and exit mechanics (drag-along, tag-along, put options, call options). A well-drafted shareholders agreement maps these explicitly.
For co-investment structures specifically, the key question is decision authority at deal level. Who decides when to enter? Who decides when to exit? Who can trigger a capital call, and what happens if an investor cannot meet it? These governance questions need to be answered before capital goes in, not when a decision needs to be made under pressure.