United Airlines' checked baggage allowance permits Economy and Basic Economy fare passengers to check into the hold bags that measure no more than 62 inches (158 cm) in total and that weigh no more than 50 lbs (23 kg).
The economy class is used in the same way all around the world. To get the cheapest prices, travelers usually book economy class tickets in advance. As economy class tickets are high-demand tickets, you might want to book it in advance to freeze the price in case the price increases.
Generally, first class is more expensive than economy class, though the actual difference in price depends on the route, when you purchase the ticket and how many seats are available. Economy class seats are fairly standard, while you can experience a variety of seats in first class.
In general, first class is the highest class offered, although some airlines have either branded their new products as above first class or offered business class as the highest class.
Economy Plus is an excellent option for travelers looking for a higher level of comfort but do not want all of the features of a full business-class seat. The economy plus cabin debuted on United in 1999 and is now offered on all mainline aircraft and almost all United Express regional jets.
Instant upgrades for all Premier members on Y and B full-fare Economy tickets and for Premier 1K® members on M fares are available on all flights that are eligible for Complimentary Premier Upgrades.
So while you shouldn't expect a free upgrade on a flight, you can still ask for upgrades from booking till boarding — and you may be able to pay your way to a much more comfortable seat.