Well, it matters because airlines have different rules about what you can and can't bring on board. Some airlines are super strict about the size of your cabin bag, while others are more relaxed. It's like the difference between a strict teacher and a cool teacher - you want to make sure you're not getting in trouble, right?
Carry on baggage - Viajessindestino.com
So, if you're planning a trip, it's a good idea to check with your airline to see what their rules are. Are they cool with a cabin bag, or do they have specific carry-on requirements? It's always better to be safe than sorry, right? You don't want to get to the airport, only to find out your cabin bag is too big or too small.
