the two kinds of limits that can force a story to its climax
Stories would go on forever unless they were limited in some way, forcing an end to action and/or decision. One way to bring a story to a conclusion is with a Timelock which limits how long the characters have to solve the problem. The limit might be a bomb set to go off, the timing mechanism on a safe, or the poison that takes effect in 24 hours - anything that has a specific deadline and needs to be prevented or achieved. The other way to force a story to end is with a Optionlock which limits how many things the characters can try to solve the problem -- trapped aboard a spaceship with a vicious creature with no one coming to the rescue, trying to escape from Alcatraz, struggling to save a relationship -- anything that has a specific scope and needs to be resolved. So in short, in a Timelock the characters run out of time, in an optionlock the run out of options. As a side note, Timelocks and Optionlocks can co-exist but only one can be the real limit that forces the climax.