Can anyone explain to me what it means when a cuboid is extended not fully functional? Does this just mean that the puzzle can't shape shift like a C4U 3x3x4? Sorry if this seems like a dumb question I'm still relatively new to Cuboid lingo.

Extended means that the puzzle is essentially a shape-mod of a simpler puzzle. For example, an extended 3x3x4 is just a 3x3x3, but it has extra pieces glued to one face to make it look like a 3x3x4. A fully functional 3x3x4 (like the c4u 3x3x4) would work like you would expect it to (and that means it actually wouldn't shapeshift).

Ah, that helped alot. Thank you very much.

If someone ever says cuboid it means a fully functional puzzle. It doesn't say anything about shape shifting yet though. A fully functional 3x3x4 doesn't change shape simply because a 90° of a 3x4 face obviously leads to a block of the mechanism. So cuboid can change shape but it depends on the dimensions.
There are also exceptions. For example there are 3x3x2 cuboids, I believe invented by smaz, which can change shape. I don't really know how.

The domino existed long before smaz's version I believe. I would also argue that if it can do that funny move you're talking about, that it is not a true cuboid.

Just to add to the confusion sometimes people make fully functional cuboids by extending other puzzles. For example, the common way to make a 3x3x5 is by extending a 3x3x3. In this case though the extensions are fully functional. Normally though as stated if a puzzle is labelled as 'extended' it does not have full functionality.