When using the scoreboard command: "/scoreboard players reset" You can hit tab, and all the players who have been in said world (even ones who are not online) are able to be selected. If an individual player is specifically chosen (/scoreboard players reset Bezatrix), the player's scoreboard scores will be reset, regardless if the player is offline or online. But, if you do the same thing, only using the selectors such as @a, @r, @p, (/scoreboard players reset @a), only the players who are online at the moment in the current world, will be affected. if you do a scoreboard reset, then a player re-joins the game afterwards, they could have a different score than the players who score was reset.