From the help page:

Call Bullpen - This value refers to the situation at which you would like your manager to take out a pitcher. This setting functions in a similar manner to the team managerial settings where the scale is 1-5, with a one indicating the least aggressive/rarer option and the five the most aggressive/frequent option. In this respect a pitcher will be removed at the first sign of danger if his call bullpen setting is set to five. If his Call Bullpen setting is set to 1 a manager will do his best to allow the pitcher to pitch out of a jam and reach the player's target pitch count. To edit this value, please click on the dropdown arrow for the player under Call Bullpen and select one of the values listed. Upon submitting pitching roles in the Edit Pitching Staff page, the recommendations will optimize Call Bullpen settings based on the player role and his pitching ability in respect to winning individual games. It will not optimize for player development, individual durability of the player or the durability of other pitchers on the team. For instance, the recommendations will list a poor pitcher or pitcher with a poor makeup, temper and/or patience ratings as a four or five for his Call Bullpen setting. If this pitcher has the highest stamina on the team and must eat innings to protect the rest of the staff, you may want to change his Call Bullpen setting to a one or two. When a situation calls for a relief pitcher, the AI will assess pitcher roles first and then availability in choosing the replacement.