A caption is a paragraph in the Caption style holding a sequence field — one counter per category, in document order, renumbered as the document is edited and on load (a file's cached numbers are unreadable wherever the format is not arabic). Round-trips as an ODF text:sequence with LibreOffice's own ooow:Illustration+1 formula and a Word SEQ field. Caption style values probed off LibreOffice: 10pt italic, 0.212cm around.