Page positions and page labels are different things. Every PDF page has a position: first, second, third. A page label is an optional display name the file can store for a page, and viewers that support labels show it in the page box and under thumbnails instead of the position. The PDF format defines labels by ranges: a range starts at a given page and can carry a numbering style, a text prefix and a starting number. That design fits books, where front matter runs i, ii, iii and the body runs 1, 2, 3.
Why sheet numbers need one range per page. A drawing set has no arithmetic sequence to follow. A-101 might be followed by A-102, then A-201, then S2.01, then M-1A. SheetStamp therefore starts a new range on every page and gives it only a prefix, the sheet number itself, with no numbering style. The label is the literal text from the review table, so A-101 shows as A-101 and S2.01 as S2.01.
Pages without a number. If a row has no sheet number when you save, that page's label is its plain page number, so the page box is never blank. Such rows are flagged as Sheet number not found before you export, and typing a number in the Sheet cell gives the page a proper label.
Where labels show, and where they do not. Viewers that honor page labels display them; we confirmed this in a full desktop PDF reader. Browser PDF viewers, including the one in Microsoft Edge, ignore page labels and keep showing 1, 2, 3. That is how those viewers behave, not a fault in the file, and the bookmarks SheetStamp writes alongside the labels still appear there. If the people you send the set to read it in a browser, the bookmarks are what they will use.
Replacing old labels. Some files arrive with labels left over from the program that assembled them. SheetStamp writes a complete new label set into the copy, so old labels are replaced rather than mixed with the new ones. The one-file-per-sheet export is different: each split file is a single page with a bookmark and no label.
Read before you write. Labels are only as good as the Sheet column. Windows text recognition can misread or drop short numbers such as T1, and a number misread in the usual spot is flagged only when it duplicates another sheet. Look down the Sheet column under All pages before choosing Save bookmarked copy.