How to draw a graph describing cases of some functionality?

Question

Analytics

Briefly. There is some business logic object with which you can perform some actions and which depends on the state of other business logic objects. There are things that the customer specifically wants, besides this, there are obvious and obvious things that can happen, and everything else is no longer so obvious and is not implemented due to the fact that such cases simply did not come to mind.
As a result, later, upon acceptance, or already during operation, these cases come out, cases in which something should happen, but does not happen, because it is not laid down.
The solution here, I see the use of some software, in which a list of entities is entered, their relationships with each other, a list of possible actions with them and their states, events, etc.
And then a graph is built, and a list of cases that need to be foreseen and implemented is issued. Is that what is on the market, or free (of course, free is better, you don’t have to beat out the budget), which allows you to solve such problems?