A release clause sounds like a single mechanism and behaves differently depending on where the contract was signed. In some countries it is compulsory, in others it is a negotiated extra.
Mandatory buyout figures
Certain jurisdictions require every professional contract to state a sum at which the player may unilaterally terminate it, on the reasoning that no one should be bound indefinitely.
The figure is deposited with the league rather than paid to the club directly, and the process is initiated by the player rather than by the buying club.
Clubs respond by setting the figures far above market value, which converts a protection for players into a headline number that is rarely triggered.
Optional clauses elsewhere
Where no such requirement exists, a release clause is simply a term one party negotiated, and many contracts contain none at all.
Players with strong bargaining positions secure them, often limited to offers from abroad, from clubs in continental competition or above a specified sum.
Because they are negotiated individually, the conditions attached vary enormously and are rarely reported accurately.
Conditional clauses are the common form
Many clauses activate only in defined circumstances, such as relegation, failure to qualify for Europe or an offer arriving within a particular window.
Others specify that the club must inform the player of any offer above a threshold without obliging it to accept, which is a much weaker provision.
Reports describing a player as having a clause frequently omit these conditions, which are the part that determines whether it can be used.
Why a clause is not always good for a player
Triggering a clause removes the selling club from the negotiation, which also removes its incentive to facilitate the move on favourable personal terms.
The buying club has paid a fixed sum it may consider excessive, and that affects what it is willing to offer in wages.
Negotiated transfers frequently produce better outcomes for everyone, which is why clauses are often used as a starting point rather than exercised.
What this means for reading a rumour
A reported clause figure tells you the ceiling of a negotiation and almost nothing about whether a transfer is likely.
The relevant questions are which country's rules apply, what conditions attach and whether the buying club can accommodate the wages afterwards.
Without those, a clause is a number in a headline, which is generally why it was mentioned at all.