I kind of forgot about this thread.
Actually 2nd's Spelljammer ships can enter the flow using Phlogiston which allowed them to fully cross entire campaign settings, like moving from Dragonlance to Faerun.I was thinking of 2E here, when the three featured campaign settings Faerun, Krynn and Oerth were planets within the Material Plane, and the Material Plane also consisted of the space, the phlogiston and the celestial bodies between these planets.
Spelljammer's setting is pretty much fifth dimensional traveling explained in three dimensional terms for simplicity. Claiming Faerun, Krynn, and Oreth all float inside the same Material Plane isn't very accurate.
The planets Sirion, Reorx, Chislev, Zivilyn, Krynn, and sixteen different moons float in wildspace (aka space) and they are all enclosed in a crystal sphere known as Krynnspace that floats in a sea of phlogiston. You can physically travel to each of those planets, like a Beholder's levitation allows him to float clear out of Krynn's atmosphere and he can slowly float to another planet, but there are only five ways to leave the crystal sphere.
Assuming you can open a portal through a crystal sphere you'd enter the phlogiston (also called the flow), an extremely flammable and incomprehensibly huge fluid-like area, where density of the rainbow ocean varies but unlike wildspace it isn't an empty vacuum. It's possible to travel to another crystal sphere known as Realmspace. This crystal sphere contains several planets including Anadia, Colair, Karpri, Glyth, Chandos, Garden, H'Catha, and Toril which is the home planet of Forgotten Realms. And Oerth, aka Greyhawk, is instead another crystal sphere known as Grayspace with a dozen other planets I'm too lazy to type in.
The Flow has a number of rule changes tied to it but the short gist is divine magic fails (DMs may allow 1st~2nd level spells through), you cannot use anything that calls another creature or anything that requires another Plane, anything fire based tries to blow up in your face, and pretty much every teleportation effect cannot be used to travel from one crystal sphere to another and they simply fail in the flow but some specific teleportation-based Spells can be used to open portals allowing you to physically travel through a crystal sphere barrier. All of this is because phlogiston exists outside of any campaign setting's cosmology and even outside the reach of it's deities.
Think of it like in 3.5's default cosmology the Astral Plane is the space between the planes and every Extra-Dimensional space, Outer, Inner, Transitive, and Material Plane floats in the Astral Plane's infinite area. In Spelljammer, The Flow is the space between entire cosmologies and every campaign setting, it's Planes, and it's planets floating in space are housed in a single easily identified sphere that floats in the phlogiston's infinite area.