(as what the title says.)
There’s no automated functionality for this. Look through and calculate the page ranges manually, then type into the Page Range box in the right panel of Print mode and export repeatedly, each time with a different range that corresponds to a different flow.
Or export once and split it in any good PDF editor.
Or (he said, pre-empting likely follow-ups), if you can guarantee that you’ve done
no manual tweaking with the properties scope switch set to Local
made no adjustments to Layout Options that you’ve not saved as default
added no system breaks, frame breaks, condensing changes, note spacing changes
changed nothing in staff spacing mode, note spacing mode or frames mode
you could create new score layouts that each only contain one flow, then export those. The listed caveats are potentially huge (and my list isn’t exhaustive) so in the vast majority of cases I can’t recommend this method.
If you specify each page range separated by a comma, they should export as separate files, meaning if set up correctly for that layout, you could export once and produce a separate file for each flow. (You’d want to make sure you choose “create new” not “overwrite”, as the file names will be the same.)