This feature allows you to download a file in order, from beginning to end, instead of the usual (and sometimes faster) random order. This can be helpful if you want to start viewing the file right away, before the download completes.
In the main transfers view, simply right-click any file in the bottom Files tab. From the menu, go to the Priority sub-menu, and at the bottom will be a checkbox to activate sequential downloading for that particular file.
You can adjust some of the parameters for sequential downloading, such as aggressiveness, head and tail first mode, and limits based on the number of seeds in the torrent. Go to Settings > Transfers > Files and click the Configure button where it says "Sequential download options". A new settings popup will open.
You can also have automatic sequential downloading for certain files types or files matching a wildcard. This can be done in the File Auto-Select
feature, which is also available from Settings > Transfers > Files.