standard way: they are folded, with a tab or raised edge to write on. However, machinery to pre-score lines in an envelope-style file is less common in the UK. Consequently, English archive folders are typically stiffer than those in the US or Canada with fewer or no squared-off bottoms. Traditional flat-style folders can be found in UK archives, but more recently, UK vendors produce punched-tab folders like the ones in the photographs below. B. Basic configuration Horizontal (flat) storage is a key distinction between the UK and North America. In England, hand-scale documents are generally stored flat within boxes, a style that often accommodates UK-based document sizes rather than the international standards commonly used in North America. UK archivists refer to a file of hand-scale documents stored within a box as a ‘bundle.’ (To a North American archivist ‘bundle’ might suggest tying documents together with string!) The box that we North Americans orderly pot of file folders call a bundle in the UK. This box contains flat files of documents, in contrast to upright files. Packaging placed horizontally, as seen in the photo above, follows some of the principles found in vertical packaging, but has specific advantages and drawbacks: Horizontal packaging differs from the vertical approach
https://peelarchivesblog.com/2024/09/10/how-do-archivists-package-things-the-battle-of-the-boxes/