Dimension 26, alternately called the 26th Dimensional Way, is Sub-Space. Not be confused with Desynched space.
Dimension 26 is basically a warp tunnel that can lead to any dimension at any year.
The fifth stage in R-Type Final leads to the pilot through a visually warped space where a Fine Motion lies in wait at the end. Depending on which components are destroyed in the ensuing battle, the pilot may end up in several other stages.
Little known feature: In this stage, the amount of distortion is controlled by your ship's speed. Since most players use Lvl3 and 4, they may never come across this as evidenced by many playthrough videos. Setting your ship's speed to 1 will make the level almost normal.