I'm really confused about this, tried to read a lot about diagrams but I just can't understand what is most suitable for this case.
I need just TO SHOW that I have 10 modules, which have 10 features. One of that feature can call other module feature..
something like this. very simple. it's better just to be some kind of blocks with depedencies[as features] and how each module block will interact with another block
purpose is to show what open source system doesn't have, and what this system WILL have after modification. And ofcourse with colors, it's easy to display all modifications.. than it's easier to start planning a project and count how much hours will be spent on things you need to make..
The very reason you are confused is that the creators of UML just made a catalog of their diagrams (a dozen types) and never really tells you when to use one or the other.
If you're talking about features, it means use case diagrams. You can see some examples here: http://askuml.com/blog/login-page-use-case-diagram/ or http://askuml.com/blog/e-commerce/