I think by "Slide bar" or "slider" style they are probably talking about what RJ mentioned, those single pin HHA style sights where you can slide the back end of it up or down to preset yardages.

The slide bar sight that Arthur shows is perfectly legal for IBO or ASA Hunter class competition as long as you are using a fixed pin aperture and don't adjust teh gang on the course. In other words leave it locked.
There are too many hunting style sights on the market right now with bars that can be slid in and out for sight plane length (Ie. Spot-Hogg, copper John, Viper, Sword etc) for that to be what they are talking about.
These are the MOST popular type sights for hunter class competition.

Then again I could be wrong ........ but I'd bet a $1 that by "Slider" they mean the single pin adjustable type like an HHA Optimizer etc. If they meant the other 3/4 of the people who shoot Hunter class 3D in my area wouldn't be able to shoot there.