For true serious low-budget and in my opinion crap, airlines, like Spirit what I am about to say is not true but for all the others that I have encountered it seems as though they are using tricky wording in an attempt to get people to pay to check luggage.
I took the above image about a very informative article about the difference between a "personal item and a "carry on bag". They do a real good job of explaining the difference but these days in my travels I have found that there are many airlines, low-cost airlines in particular, that are intentionally tricky about how they word things which I believe is in an effort to have you trapped at the gate and forced to pay some extortionate amount of money to get your small bag on board.
I have flown with Spirit Airlines exactly one time and will never do so again because of this horrible policy that is all about getting money out of people. If you want to have a good laugh search YouTube for videos of people losing it with Spirit Airlines. Sometimes a low price is not the best thing and nor is it going to be the best price because come on now, who travels with just a tiny Hello Kitty backpack?
Recently I booked a flight with a lower-cost, not proper shite low-cost like Ryanair or Spirit, but the wording on the itinerary said that I was allowed a personal item. This made me angry because in the past I have used this airline and traveled with small rolling suitcase as well as as mall laptop backpack and there was never an issue.
When I dug deeper I discovered that they are using the word "personal item" to refer to carry-on bags. This means that my normal rolling suitcase it going to be just fine. So why did they use that word that has been defined by so many other airlines to mean a very small bag?
I have no proof of this but I think it is a ploy to get people thinking that they have no choice but to check (and pay for) baggage to their destination. I will do no such thing but rather than just assume that I am right it is always wise to check directly with the airline that you are dealing with, which is what I did.
I do find it very dodgy that they hide this information behind several clicks though before you can find out what I did.
The 5kg limit is ridiculous but I have been on hundreds of flights and only had my bag weighed for carry on one time and that was with Spirit who I will never use again. I would rather take a bus.
So if you are looking at a itinerary one day in the near future and you see "personal item" as your only allowable carry on bag, dig a bit deeper, because I think airlines are intentionally being tricky with this in order to make more money.
Honestly, for me the days of the low-cost airline are behind me. The "normal" airlines are only about 10-20 percent more expensive and they are much nicer and have more traditional policies and kindness that lead us all to want to fly places in the first place. I find it quite upsetting that they lured us all in with the promises of low prices, only to turn the flight experience into a massive hassle that simply isn't worth the little bit of money that you are going to save by flying with an inferior airline.