Tools for Modelling poorly documentated Legacy Systems

You have asked 3 questions with basically the same topic.  I have a legacy application with undocumented source code and need to bring it up to date.
https://www.experts-exchange.com/questions/28604469/Strategies-and-Tools-for-rewriting-refactoring-of-a-software-application.html
https://www.experts-exchange.com/questions/28607487/Assumption-Management-Software.html
https://www.experts-exchange.com/questions/28604469/Strategies-and-Tools-for-rewriting-refactoring-of-a-software-application.html
which the accepted answer in the last question has basically covered the points needed

No disrespect but not exactly the previous questions were asking about strategies/tools to attack the problem (legacy application with undocumented source code). The strategy portion of this question was answered in an essay style, so I didn't want to push that expert for anything further.  From the previous answer I did some further research and found that the a core issue in rewriting code, that is poorly documented is assumptions made by developers particularly in the area of business logic surrounding a software application hence the first question.

Additionally I looked into some tools that could model the current undocumented source for further facilitate some level of understanding of the current code. My two current questions are asking about specific software tools to assist in assumption management and modelling source code. For modelling the source code I have already found two tools just wondering if their was anything else I should look at.

Thank you, please let me know if you want me to clarify anything further.

As always any advice is welcome.

Cheers for the info.