Building a competitive Major League Soccer team isn't just about finding talented players. It's about fitting them into a very specific structure. Every MLS club's professional roster is comprised of up to 30 players. All 30 of these players are eligible for selection to the 20-player game-day squad during the MLS regular season and playoffs.

These 30 roster spots are divided into three main categories, each with its own rules and purpose. Think of them as different floors of an office building. The top floors have the senior executives, the middle has experienced staff, and the lower floors house the interns and newest hires. Each group has a different role and impact on the company's overall budget.

For an MLS team, these categories are the Senior Roster, the Supplemental Roster, and the Reserve Roster.

The core of any team is its Senior Roster. These are slots 1 through 20. Players in these spots are typically the team's starters and key contributors. Their salaries count against the club's salary budget, which is a league-wide cap on spending for player compensation.

Not all 20 slots must be filled. A team can choose to leave some senior spots open, providing flexibility during the season to sign new players. However, a minimum of 18 senior slots must be occupied. These players form the backbone of the squad, and managing their contracts and budget impact is a primary job for any General Manager.

Beyond the core 20, teams have additional spots for developing players and adding depth. Slots 21-24 make up the Supplemental Roster. These are often younger players, draftees, or those on lower salaries who are developing their skills. Their salaries do not count against the club's main salary budget.

The final spots, 25 through 30, form the Reserve Roster. These slots are reserved for players who are on a Reserve Minimum Salary contract. This is typically where you'll find players signed from a club's development academy (Homegrown Players) or very young prospects. Like the Supplemental Roster, their salaries are exempt from the team's primary budget calculations.

Every player in MLS is also classified as either a domestic or an international player. This status is not based on their country of birth, but on their legal residency and citizenship. A player qualifies as a domestic player if they are a U.S. citizen, a permanent resident (i.e., a Green Card holder), or have been granted a specific status like refugee or asylee. For teams based in Canada, a domestic player also includes Canadian citizens.

This classification is critical because each team has a limited number of spots for international players.

Each year, the league gives every team a set number of International Roster Slots. These slots are a valuable asset. A team can use them to sign players from abroad who do not have domestic status. If a team doesn't have an open international slot, they simply cannot sign a non-domestic player.

What makes these slots particularly interesting is that they are tradable. A team with an open international slot can trade it to another team in need, often for a player or for different types of Allocation Money. This creates a dynamic marketplace within the league, where a team's ability to manage its international slots can directly impact its ability to improve the squad.

Teams can acquire slots for a full season or for shorter periods. The trading of these slots is a constant strategic element for front offices. A team with a young, domestic-heavy roster might trade away its international slots to accumulate other assets, while a team with championship aspirations might trade for extra slots to bring in experienced talent from overseas.

This system ensures a balance of domestic player development while still allowing clubs to attract and sign talent from the global soccer market.

Understanding these roster rules is the first step in assembling a team that can compete for a championship while adhering to the league's complex structure.