A good answer, but not quite precise:
If you only play part of a chapter with a character, you can return to the same chapter with the same character with a different group. This rule prevents you from being effectively forced out of campaigns like Storm King's Thunder and Out of the Abyss, where a chapter can cover most of a tier of play. However, you are not allowed to participate in any encounters that character previously played within that chapter. If for some reason you must participate in an encounter you previously replayed, your character receives no reward for doing so (as the character already received awards previously).
So, if you played a number of sessions of Storm King's Thunder with a group who chose to travel to Bryn Shander, but didn't level high enough to go to the next chapter, you could return with a different group to the same chapter of Storm King's Thunder as long as that group wasn't also travelling to Bryn Shander. Likewise, if you participated in the encounter in Out of the Abyss where your group received the intelligent sun sword, and had to return with a different group to the same chapter of Out of the Abyss, your character would not be able to participate in that encounter.
I'd like to be able to link to an explanatory admin post saying the same thing, but can't find it. However, everything noted above follows pretty logically from the FAQ entry on 'Can Parts be Replayed' (FAQ 7.1, p.13); a group can agree to return to complete an unfinished chapter or (DD-series) adventure, but cannot replay any encounters previously played.
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Pauper