The Space effect is actually a parameter of the Output block. It cannot be assigned to the Stomps or Effects slots or saved per rig.
Designed to simulate a small room, this effect provides early reflections with stereo variations to add space and width to the tone.
This works well to eliminate the artificial-sounding dryness of a close-mic'ed profile when using headphones (particularly isolating headphones). Use it as a temporary global effect for such scenarios. If you have rigs where you want a similar effect, use the EQ Widener and Reverb instead - you'll get more control and can save the exact settings per rig.