The post is drawn on top of the tread and the support is built below the tread. The staircase will then look as if the post is going through the tread. However, the two parts are completely separated by the tread.
Via the predefinitions, it is possible to automatically assign drilling patters above and below the tread for this function. Alternatively, it is possible to set the drillings manually during the design process in the post properties.