The QBox, QTube, QGrid, and QSphere tools are designed to create shapes from quad polygons. When transforming, such primitives try to keep the topology uniform to avoid stretched polygons. All of these tools use the QPrimitive HDA, which is optimized for SubD modeling. Such a primitive has a global resolution parameter that can be changed with Parm Slider.
Tube and Circle tools is for creating classic primitives of corresponding type.
As a Houdini user, you should be aware that primitives can be created in separate containers, or in one shared container. This is controlled by the **Create at Object Level** and Create in Context viewport options. If the first option is active, then each new primitive will be created in a separate container. If the second option is active, then the primitive node will be added to the current SOP container, while the previous active node will be marked with the Selectable Template flag. In the same way, primitives are created in Modeler, but the last active node is marked with the Template flag, instead of Selectable Template. Usually, if you are working in the same container, after completing the setup of new primitive, you can merge different branches of the tree with the Merge Nodes tool.
Usually primitive is created in the center of container. But often, to speed up modeling, you need to immediately create a primitive in a given place and with the right orientation. There are two ways to do this in Modeler: