Terran -- this is an annuity product, but the challenge is that the 457 plan looks like it can ONLY be invested in annuities, to create a pension-like cashflow at retirement.
So, some definitiions...
A variable annuity is a tax-deferred retirement vehicle that allows you to choose from a selection of investments, and then pays you a level of income in retirement that is determined by the performance of the investments you choose. Compare that to a fixed annuity, which provides a guaranteed payout.
-- A variable annuity will move up and down with the investment... but the standard one is likely restricted to conservative investments that will always pay out a return in your 457, and it has M/E costs reducing returns a bit, too. Think of this like a bond fund?
To combat the low returns of the regular variable annuity and possibly increase the range of investments and returns available, while conforming to the "must be an annuity" in your 457, the company is offering a second product the SIO.
This is invested entirely in equities, with a guaranteed return between two numbers.
There are still usually some costs with the SIO, but if you have a very large pooled 457, these management costs may be waived / paid as an annual fee by the employer to maintain the plan, or something else. The managment company does get the extra increase in the plan above a certain number, so that is profit for them.
Ask what the SIO is invested in, what the caps are, and how the SIO's administration costs are paid / funded.
I am not sure if I would go with the SIO, unless you have the "large plan" advantage and minimal fees and you like the investments.