Firstly, you need to check if you are taking any light aircraft flights – and if you are, check what the luggage allowance is. Airlines can be very strict on baggage limits, so it’s important to double-check your limit before you travel.
If your itinerary states that soft bags are needed, this is because storage space on light aircraft flights is limited and therefore luggage should be soft (e.g. a duffel bag) so they can be squashed and manipulated into a small hold in the plane. Your bag should have no spines, frames or a rigid structure, although one flat side is usually allowed. Also, bags should have very small wheels or preferably, none at all. Your main bag will be placed in the hold, and you will be holding your carry-on bags (i.e. hand baggage) on your lap for the duration of the flight as the aircraft do not have under-seat storage or overhead lockers for stowing bags.