An IUL doesn't hold stocks. It reads an index, applies some math, and credits your cash value with the result. The math runs through three possible levers, and once you can name them, any crediting method a company offers becomes readable.
The cap is a ceiling. If your cap is 9% and the index gains 12%, you're credited 9%. Index gains 5%, you get the full 5%, since that's under the ceiling. The participation rate is a percentage share. At 50% participation, a 12% index year credits you 6%. The spread is a haircut off the top. With a 2% spread, that same 12% year credits 10%, and a 2% index year would credit zero. Same index, same year: three levers, three different outcomes. Every product mixes and matches, some strategies use a cap alone, others pair a high participation rate with a spread, and the brochure names rarely make the arithmetic obvious.
Underneath all three sits the part that doesn't move: the zero floor. A negative index year credits you nothing rather than a loss, which is the trade the whole product is built on. You give up some of the up in exchange for none of the down, and we walked through how that reset works each year in The Annual Reset and the Zero Floor.
Something to note: the levers move. Caps and participation rates are declared by the company and can be changed on existing policies, usually annually, within limits the contract sets. Your contract guarantees a worst case for each lever, maybe a cap no lower than 3%, and everything above that guarantee is current practice, not promise. A policy sold with a 10% cap can be an 8% cap policy a few years in. That's not a scandal, it's how the product is built, but it belongs in your expectations, and it's a fair question to ask about any company: how have they treated the caps on policies they've already sold?
This is also why illustrated numbers deserve suspicion. An illustration projecting decades at today's levers is projecting that nothing changes, and something usually changes. Read the guaranteed column first, the way we practiced in How to Read a Policy Illustration Without Getting Fooled, and treat the rest as one possible future. If you're comparing designs, our design page covers what matters beyond the levers.