This has to do with the breadth and variety of intentional contents, i.e., the variety of types of things intentional states can refer to, for instance, the variety of types of goals, objects, problems, plans, actions, environments etc., with which an individual can cope, including the ability to deal with new situations using old resources combined and transformed in new ways.
Flexibility in this sense is required for understanding a sentence you have never heard before, seeing a familiar object from a new point of view, coping with an old problem in a new situation, and dealing with unexpected obstacles to a plan. A kind of flexibility important in human intelligence involves the ability to raise a wide range of questions.
A desirable kind of flexibility often missing in computer programs is `graceful degradation'. Often if the input to a computer deviates at all from what is expected, the result is simply an error message and abort. Graceful degradation, on the other hand, would imply being able to try to cope with the unexpected by reinterpreting it, or modifying strategies, or asking for help, or monitoring actions more carefully. Instead of total failure, degradation might include taking longer to solve a problem, reducing the accuracy of solution, reducing the frequency of success, etc.
One of the factors determining the degree of flexibility will be the range of representations available. A system that can merely represent things using a vector of numerical measures, for example, will have a narrower range of possible intentional states than a system that can build linguistic descriptions of unlimited complexity, like:
the man
the old man
the old man in the corner
the old man sitting on a chair in the corner
the sad old man sitting on a chair with a broken leg in the corner
etc.
so flexible control systems of the future will have to go far beyond using numerical measures, and will have to be able to represent goals or functions, and relationships between structures, resources, processes, constraints, and so on.