“Dimensions”, “realms” or “'verses” could be considered as separate instances, timelines or even continuities of the overall Pokémon multiverse that are separate from each other. An example common in canon is the distinction between Mainline and PMD, where even some foundational mechanics are different.

Frankly any number of terms could be used to identify them as there is not a precise definition even in canon. For ease of use, “Dimension” or “Realm” are used always capitalized when taken with this semantic context in mind.

Overall description of the “dimensions” that can exist in the Suocéverse.

• Mainline Dimension: Also called Mainline Universe, it is the default “trainerverse” and follows after the settings seen in the mainline games and anime Pokémon franchise. The Mainline World is the Earth-like planet or world that has the focus of most of the adventures we see in this 'verse, and is the one where eg.: the regions of Kanto etc…, exist.
• Distortion Dimension: The canon “Distortion World”, an analogue but much emptier and more chaotic dimension where Giratina resides.
• The canon Dream World.
• PMD Dimension: It is the realm we see in the PMD games, that is, the Mystery Dungeon World from the Explorers, etc… games.
• The dimension where Project Nrafhal exists.
• At least one dimension following the concept of the anime's Mirror World or Futurama’s negation world - ie.: a largely similar universe except that some choice outcomes are reversed.
• An unnamed dimension with Pokémon civilizations but no humans, where adventures like Blink, Slowly are hinted to take place.

The rest of the list starting at this point exists mostly for completeness, it’s unlikely ATM (Aug 2023) that they’ll feature onscreen.

• The dimension where Ghost Pokémon are stated to live in anime / manga canon.
• The zeroth dimension or “Museum Dimension”, where Arceus is said to reside. Hosts a sort of “Earth Prime” as well as the Hall of Origin, Cave of Challengers and a sort of mesohistory-scale Safari Zone with mons extinct or even yet to exist, hence the name. Probably take clues from world of Limbus in FFXI for its design. This world is normally only accessible by the Lake / Creation Trios, or by use of the Azure Flute.
• One alternate dimension with a fundamentally different topology - such as the “planet” being flat and endless this despite it actually floating in space around the Sun and having days, nights and seasons. Something closer to a “Minecraft world” (except not blocky, and hopefully without Creepers).