Does this mean they can leave part-way, arrive part-way, or both?Characters can miss out. Players can just make more characters.
For example, you could start ToA chapter 3 and attend sessions 3, 4, and 7. You would miss some content with that character but you can come and go at will. You would not be able to run Chapter 3 with any other group with that character, even if it was content you had missed with the other group. Chapter 3 is 'locked out' for that character.
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
The first part of the quote seems to contradict your version.
"A group of characters that play only part of a chapter or adventure can’t return to that chapter or adventure unless the entire group agrees to do so. That is, if a game runs exceedingly long and all players and the DM agree to meet the following weekend to pick up where they left off and finish the game. If this occurs, appropriate entries should be entered on logsheets to reflect this.
Replaying the entirety of a portion of an adventure you want to replay is not permitted.."
It really makes no sense to restrict a group of characters but then allow an individual character to return to a chapter. If there was a later change or clarification, they should update the FAQ.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.