With manual substitutions, you can directly sub out players in your team and replace them with players from your team that are yet to play.
Between matchdays, you can make an unlimited number of substitutions as you play around with your squad.
But once the matchday had started, you can sub out a player from your starting XI (maximum 4 players) and replace him with one of your bench players who is yet to play.
Of course, points of players you sub out will not count for your team total on that matchday.