When a Future Reality Tree is complete, it will contain several (even, numerous) "Injections." An Injection is a condition that simply has to exist in the future, in order for the symptoms identified in the Current Reality Tree to be eliminated. For example, "Our time to get an estimate to the customer is never greater than 3 days" could be an Injection.
The FRT is not complete until all ther major Injections have been identified; it is these that convert a good idea to a good solution.
However, the Injection is something that will HAVE to exist. The FRT doesn't say a lot about how to put it in place.
Sometimes, it's obvious. But sometimes it isn't; you can see some serious obstacles that will stand in the way of getting that Injection into place.
That's where the Prerequisite Tree comes in to play. An excellent management tool, it starts with a clear statement of the Injection that will have to be in place, helps a manager to surface and articulate the major obstacles, and to decide on ways around, through or over those obstacles. By the time the PRT is completed it's a road map of how to get from "now" to "then."
The PreRequisite Tree is a planning tool that actually uses Obstacles as levers to achieve the end result
Often, the obstacles to achieving something appear to be so large as to make it appear beyond reach. Or at least, to make the path so difficult that it is not easy even to see where to start. Both of which are primary causes of procrastination, of course.
The Prerequisite Tree is a device for forcing out all the obstacles, and identifying ways around or through the obstacles, with the outcome being a clear-cut roadmap to follow.
The beauty of the approach is that there are always obstacles to achieving anything worthwhile (or else it would already have been achieved, of course); and, there are always people skilled in seeing these obstacles, to the point where they might not even be able to see the benefits of a solution because all they see are the obstacles they face.
This tool makes the obstacles a vital part of the solution itself, and makes the people who are often portrayed as "negative nellies" a vital part of constructing the solution.
Teams find this a particularly useful tool, depicting the big-picture of how they are going to get where they are going, what all the intermediate steps are, what the obstacles are, how they are going to overcome them, and how all the elements come together to provide the solution.
With this Theory of Constraints Thinking Process there is never any ambiguity as to who is responsible for what.