Sequence and Act, and How They Differ
A sequence is a group of scenes that all move the same value in the same direction, the way a chapter might follow one goal through several attempts before it either succeeds or fails outright. An act is a larger container still: a stretch of the book, usually one of three, that ends when the whole situation has genuinely changed, rather than one small piece of it. Sequences sit inside acts the way beats sit inside scenes, each level a slightly bigger version of the same question: what actually shifted here.
Where the confusion comes from
Acts get reduced, often, to a position in the manuscript, as if Act One simply ends at a fixed point and Act Two simply begins after it, regardless of what's actually happened by then. That treats an act as a measurement of length rather than a description of change. The three-act shape itself isn't a formula somebody invented and enforced. It's closer to a pattern noticed after the fact, in stories that already worked, and it describes what a beginning, a complication, and a resolution tend to do to each other, not where the page breaks should fall.
How this shows up in a real novel
Around the World in Eighty Days (Jules Verne, 1872) is built from clear sequences: each leg of Phileas Fogg's journey is its own small arc, complication rising until it nearly costs him the wager, resolved just in time for the next leg to begin. Those sequences accumulate into a single act-level shift by the novel's end, when Fogg believes he's lost everything by one day, only to discover, through the international date line, that he was wrong the whole time.
While you're drafting
Sequences are usually easier to feel than acts while a draft is still being written, since a sequence rarely runs longer than a chapter or two and its own success or failure tends to be obvious on the page.
Questions
Is a sequence the same length as a chapter? Not necessarily. A sequence is defined by the value it moves, not by a page count, so it can span less than a chapter or run across several.
How do you know an act has actually ended? Not by a page number. An act ends when the overall situation has genuinely changed for the people in it, not simply because a certain amount of the manuscript has gone by.
Do all novels use exactly three acts? Most describable stories can be read that way in hindsight, but the three-act shape is a pattern noticed after the fact, not a rule every novel was written to obey.