You can choose the method by which you would like to join the duct. Selecting 'flanges' will mean that the duct is supplied with ready fixed 'Doby' flanges on either end. These flanges enable the duct to be joined by simply bolting it together - one M8 nut and bolt on each corner. Nuts and bolts are available in the Fixings & Supports section.
Selecting 'flanges loose' will mean, as is suggested, that flanges will be supplied but not fixed to the duct. They can be fixed on with rivets or screws as part of the installation process. Situations where this may be useful are if the duct needs to be trimmed in length to fit a confined space, or where a duct needs to be pushed through a hole or gap and pre-fixed flanges would make this difficult.
Alternatively, selecting 'slip-joints' will mean the duct will be supplied with the ends fashioned in such a way that they slip one inside the other to a depth of 50mm. They can then be fixed with either drill-screws or rivets, and sealed with sealant or tape.
Selecting 'nothing' will mean the ends will be plain, as in the drawing on the product page.
Lastly, there is the option of having one end of the duct length capped off. Usually - but not always - the last piece of a run of duct is capped.