Pose from Homography matrix - Zhang method
The coordinate system you choose is completely arbitrary, as no information about real-world coordinates can be inferred. From an image of a table there is no reason to know that one leg is located at any particular $(X, Y, Z)$, or that it is any particular size (you can't tell if it's a doll's table or a giant's table).

Normally you would choose one of your cameras to be located at the origin, looking down the $z$ axis, defined by the matrix:

$$[R|t] = \begin{bmatrix} 1&amp;0&amp;0&amp;0 \\ 0&amp;1&amp;0&amp;0 \\ 0&amp;0&amp;1&amp;0 \end{bmatrix}$$

Then due to the scale ambiguity you would have to choose an arbitrary scale, for example if you are using a stereo camera you could set the distance between the cameras to be one unit distance.