The duel spirit realm is a place where duel spirits reside when not being used in duels.
The duel spirit realm that Duela visits is based on her deck's field spell, Skyscraper. It's shown that its like most metropolises, with shopping, taxis, ect. It also has a distant forest, where Eatos has a citedel.
Neo Space is a distant part of this region, homeworld to Jaden's Neospacians. Ancient Fairy Dragon's region is not part of this spirit world