Clubs may make discovery claims on players not yet under MLS contract who are not subject to the allocation ranking or lottery mechanisms.Each club has the opportunity to make six discovery signings per season (expansion teams may make 10 discovery signings in their inaugural season). A club may have up to 10 discovery claims on unsigned players at any time and may remove or add players at any time. The last day for discovery player signings is September 15, 2014 – coinciding with the roster freeze date and trade deadline.The six discovery signings can be used to fill senior roster spots only. If multiple clubs claim the same player using a discovery, the club that filed the claim first will have first rights to the player. Discovery claims expire following each season. If the League and player are unable to reach an agreement during the season, the club that first filed the discovery retains the right of first refusal in the event the player is later signed by the League.Special Discovery Players: Each team may have only one Special Discovery signing per year. Special Discovery signings will occupy one of a team’s six Discovery Player slots. Teams may amortize the total amount of acquisition costs over the term of the player’s contract as long as the amortized amount and the player’s salary do not exceed the Maximum Salary Budget Charge.
Essentially this means that any player who is not in the league but one day MAY play for the league can be claimed by a team. Then if that player tries to join MLS then that team who made him one of their “discovery signings” gets the right of first refusal. For example, if Samuel Eto’o wanted to join an MLS team he would not have an open choice of who to join if a team had already made him one of their six discovery signings prior to him trying to sign with the league (When a player signs with a team he is actually signing with the league). Therefore, if Sporting Kansas City had his rights and LA Galaxy did not, but wanted to sign him, then Sporting Kansas City would be the first team able to sign him. If Sporting Kansas City did not want him then the player would be available to the next team on the list and if no other team was on the list for that player, based on the discovery signings the MLS club teams had made beforehand, then the LA Galaxy would be able to sign him.
Surprisingly, it is quite possible that a team has the “discovery rights” to Lionel Messi just in case he decided to play in the league. MLS has a lot of weird and convoluted roster rules in order to keep the league salaries low and secure in order to fix the mistakes of the previous incarnation of the NASL which paid players way too much, far more then the team and league itself was earning. This “discovery rights” rule is meant to eliminate free agency and the over-inflation that drives up player wages that come from free agency. Without several clubs bidding for a player, then the players price cannot astronomically rise which makes it easier for teams to sign new players without destroying their salary cap. The other effect is it provides balance within the league as it prevents NYC, LA, and the other teams in big cities from snatching up all the big players. At the end of the day, rules like this are important in order to make sure MLS remains financially sound, especially with the inflated prices of soccer players in Europe and the other major American sports. However, as the MLS becomes more popular and earns more money it will be increasingly more difficult to keep rules like this in place and it will be a cause for debate in many contentious CBA negotiations.
Sources: 2014 MLS Roster Rules