Settei
What is a settei, you might ask? Let's say if you and I were artists around the same level of experience and we drew Ash or Pikachu right now from memory. They'd look quite different wouldn't they? Perhaps my Pikachu's feet are longer compared to your Pikachu with short feet. My Ash might have a flat brimmed snapback while yours has a curved dad hat.
If every animator drew a character from memory, that character would look inconsistent or "off-model" throughout just one episode! That's where settei comes in handy.
Settei is the Japanese term for character model sheets. They're used by animators so no matter who the animator is, they have a reference to look at so characters still look "on-model" or similar to one another.
Depending on the animation director for the episode and animators themselves, we still get characters that are slightly off-model or extra good looking... we'll have a look at that once I watch a few episodes and make some notes myself.