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I found this rule
2-8 Safety Play Prior to any shot except the break, you may declare a safety. On a safety, your inning ends after the shot regardless of whether you pocket any ball. If you do not declare a safety and you pocket a ball on an obvious shot, your inning continues and you must shoot again. A safety must meet the requirements of Rule 1.19, Legal Shot, or it is a foul. (AR p. 77, 103)
I'm not sure I understand the rule. Why would you ever call "safety" instead of calling a random ball in pocket on the off chance that you make it?