C++ and OpenCV: InputArrayOfArrays Type and reconstruct() function in SfM Module

I am experimenting with the SfM Module in opencv_contrib, and I was reading the documentation for the reconstruct() function. I'm a bit confused as to one of the parameters that I can pass to it.

One of the arguments is:

InputArrayOfArrays points2d

and that's defined as:

points2d Input vector of vectors of 2d points (the inner vector is per image).

Does that mean we flatten the Mat data type of each image into a vector of vectors or something? I don't get it.

Do I need to use a feature matcher and find matching features, then put those features in this vector?

Either I'm not understanding what it means properly, or the documentation is a bit confusing. I'm rather new to OpenCV in C++.

Thanks in advance!