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From BR Bullpen
A player is given his unconditional release when his team no longer has a use for him and wants to terminate all of its contractual obligations towards that player. If the player is a major league veteran, he is first placed on irrevocable waivers for a number of days, allowing any team to claim him and assume the remainder of his contract. If he is not a veteran, he becomes a free agent immediately, with the releasing team paying him a portion of the year's contract, depending on the date of release.
In the days of the reserve clause, an unconditional release was the only way in which a player could become a free agent and therefore offer his services to all other teams. Teams were reluctant to release any player who had any potential value, even as a minor leaguer, and thus this fate was reserved for players who were considered to be completely washed up. Nowadays, a number of rules exist that limit considerably the number of years a player remains tied to a particular organization, and an unconditional release is not a traumatic event. It has often been used in recent years for players who are still capable of playing, but who have huge contracts that make them untradeable once their original team no longer has a need for them on its roster. Famous examples of such releases are Damion Easley and Russ Ortiz.