FLEX contracts let an institution pick terms that no listed series offers — a strike of $51.37, a five-year expiry, European exercise on an equity — while still clearing through the occ instead of taking counterparty risk on a bank.
Retail traders rarely enter them directly, but they matter indirectly: many structured products, buffered ETFs and defined-outcome funds are built entirely from FLEX options, so their behaviour is the behaviour of a vertical-spread wrapped in a fund.
Example: a buffered fund promises the first 10% of downside absorbed and upside capped at 15%. Inside, it holds roughly a one-year FLEX protective-put spread financed by a short call — a collar with custom strikes that no listed chain could match.
Related: occ, collar, european-style-option