follows the North American model, with a single premier league and no promotion or relegation. Major League Rugby, which began play in 2018 as the second attempt to launch a professional rugby union competition in the U.S., has a business structure identical to that of MLS. It is a single business entity, with each team owner-operator being a league shareholder, and all player contracts are held by the league. Also similar to MLS, MLR uses playing rules set by its sport's international governing body. However, MLR teams do not play competitive matches against teams from other leagues, and there is currently no cup competition in U.S. rugby similar to soccer's U.S. Open Cup. The MLR league structure also follows the North American model of one premier league without promotion and relegation. A more rarely seen business model in North America is the pure single-entity league, typified by the two incarnations of the XFL, where there are no individual owners or investors, with all teams centrally owned and operated by the league. Many upstart leagues begin their existence as pure single-entity leagues before they secure investors for teams (such as the Premier Hockey Federation) or are forced to operate in the pure single-entity model when investors fail to materialize (such as the Stars Football League). Many North American systems have a secondary or minor league but without promotion and relegation of teams between them. Often a minor league team is partly or wholly owned by a major league team, or affiliated with one, and utilized for player development. For example, Major League Baseball operates Minor League Baseball. The minor clubs do not move up or down in the hierarchy by on-field success or failure. Professional ice hockey has a system somewhat similar to baseball's (without as many levels), while the National Basketball Association operates a single NBA G League. The National Football League does not have a minor league system as of 2023 but it has operated or affiliated with minor leagues in the 1930s, 1940s, 1960s, 1990s, and the early 2000s and invested in one in the late 2000s. To prevent conflicts of interest, most North American sports leagues that operate on the franchise system do not allow individual owners to hold more than one franchise at once. Most such leagues also use revenue sharing and a salary cap in an effort to maintain a level of parity between franchises.