Helical staircases do not have a central post like spiral stairs and also have handrails on both sides rather then just the outside balustrade. They are also not limited to a circular shape, and may be built in an elliptical or oval style.
Because of its helical nature, it is possible to build two helical staircases while occupying the same vertical space. Fire escapes are a good example of this.
Most modern buildings, if space allows, opt for a helical design as it is the most fluid of all staircase strings and can be manipulated to take on both modern and traditional features making this type of design very diverse.