How it matters to the founder - if it's uncapped then as the next round goes to infinity, the equity from the SAFE round goes to zero (see the 200MM example). I

How it matters to the founder - if it's uncapped then as the next round goes to infinity, the equity from the SAFE round goes to zero (see the 200MM example). If It's capped, then the SAFE will convert at no lower than X%. Uncapped is better for the founder, but the difference only will ever really matter if it takes off so fast that the next round is at a much higher valuation than expected.