From the "Help" for the "Edit Player Settings" page:
Relief - Check this box for a player for whom it is ok to enter as a pitcher in relief. If this box is unchecked, the player will never come into the game in relief (unless assigned a relief role via the Pitching Staff page). In some cases, users will want to allow positional players who are rated relatively well in pitching to fill in for relief when necessary. By default and when clicking 'Show Recs', this box is checked for relief pitchers and not for any other player. Also, starters may pitch in relief for non-started games, but only if this box is checked. The game's AI will treat all players who are not set to a relief role, yet are available in relief, as the lowest priority Long Relief pitchers. Relief situations are determined by your edit pitching staff settings in respect to the game being simulated.
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