Meaning of Input Array and noArray()

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Hi

In the wiki I found the following description of a function:

C++: Vec2d EM::predict(InputArray sample, OutputArray probs=noArray())

what does it mean exactly? I.e.

a) What exact type can "InputArray" be? A cv::Mat? or also a std::vector or also a simple C-style array?

b) What does the value noArray() specify? Is this a function? Or does it simply mean that I don't have to pass this argument because OpenCV can choose a default value here?