Two people in adjacent seats can have entirely different tickets. What separates them is the fare basis, a code carrying the rules that govern everything the ticket can and cannot do.
A cabin contains many fares
Airlines divide each cabin into fare classes, identified by letters, and sell a limited number of seats in each. The classes share the same physical seat but differ in price and conditions.
Revenue systems open and close those classes continuously based on how bookings are tracking against the flight's forecast, which is what makes prices move.
When people say a fare is sold out while seats remain, they mean the cheaper class closed while a dearer one is still available.
The fare rules are the actual contract
Each fare carries rules covering changes, cancellation, refundability, advance purchase, minimum stay, routing, upgrade eligibility and how the ticket can be reissued.
Those rules, not the price, are the substance of what distinguishes tickets. A restrictive fare may permit no voluntary change at all, or permit one at a fare difference.
Because rules are set by each airline and revised over time, the conditions displayed at purchase for your specific fare are what apply.
Fare difference is where the cost hides
Even where a change is allowed, moving to a flight where only a higher class is available means paying the difference between fares, which can exceed what a new ticket would have cost.
That is why a change can be described as free and still be expensive. The fee and the fare difference are separate items.
Award tickets follow their own parallel rules, which are set by the loyalty program rather than by the fare.
Involuntary changes are handled differently
When the airline changes or cancels the flight, a different set of provisions applies, generally more generous than what a voluntary change permits.
Passenger rights in these situations are governed by regulation as well as by airline policy, and those rules differ between jurisdictions and are periodically updated.
Why this matters before you buy
Comparing tickets on price alone compares only one attribute. A slightly dearer fare in a more flexible class can be the cheaper purchase once the probability of a change is considered.
The class letter appears on the confirmation, and looking it up against the airline's own published conditions takes a few minutes at booking and answers questions that are far harder to resolve later.