In addition to the standard use of drag and drop, elements which require a start and end point to be defined (e.g. walls) will make use of left-click to place the first point (i.e. the start point for a wall) and then left-click again to set the end point. Other elements such as a staircase will require 3 placement clicks to determine size and orientation.
Many of the tools accessed from the Tool Menu Bar have placement options. This can be used to decide the placement method of item you are trying to create. If such options are available, you will see the dynamic Tool Placement Context Bar will be displayed on the inside of the left Tool Menu Bar or on the inside of the right Tool Options Catalogue. There are a number of tools that include the polygonal input method such as roofs or ground area for example therefore, the number of placement clicks depend on the type of element.