Hal Jordan's Pocket Dimension
The Power ring has multiple and almost infinite abilities, it all depends on the creativity and willpower of the wielder to imagine them and be able to use them, one of them is the creation of an entire world inside the ring that can only be accessed through willpower by the wielder.
For example, the Green Lantern Abin Sur once defeated a very powerful sorcerer called Myrwhydden, and to imprison him somewhere that he couldn't escape from, he reduced the sorcerer to a microscopic size and put him within the ring in a world created by the power ring's willpower.
Whether this created world is a Tesseract, a virtual world, a simulation or any kind of area, but it can withhold entities inside. It's still not clear if this created world exists in all the rings or if it changes or if it's a different one for each ring, or if there are rings with a world within and some that don't.