The 2D Scene Graphs
By default, there are two different scene graphs created automatically when you start up Panda3D. These graphs are referred to by their top nodes: render and render2d.
You use render most often; this is the top of the ordinary 3-D scene. In order to put an object in the world, you will need to parent it to render (or to some node that is in turn parented to render).
You will use render2d to render 2-D GUI elements, such as text or buttons, that you want to display onscreen; for instance, a heads-up display. Anything parented to render2d will be rendered on top of the 3-D scene, as if it were painted on the screen glass.
The coordinate system of render2d is set up to match that of the mouse inputs: the lower-left corner of the screen is (-1, 0, -1), and the upper-right corner is (1, 0, 1). Since this is a square coordinate system, but the screen is usually non-square, objects parented directly to render2d may appear squashed. For this reason, Panda3D also defines a child of render2d, called aspect2d, which has a scale applied to it to correct the non-square aspect ratio of render2d. Most often, you will parent GUI elements to aspect2d rather than render2d.
Specifically, the coordinate system of aspect2d is by default scaled such that x ranges over [-ratio,ratio], and y ranges over [-1,1] where ratio is screen_size_x/screen_size_y (in the normal case of a window wider than it is tall).
There is one more child of render2d to take note of, called pixel2d. This is scaled in such a way that one Panda unit represents one pixel in the window. The origin, (0, 0, 0) is in the upper left corner of the window. The lower right corner has x and z values equal to the width and -height of the window respectively. As Panda3D uses a Z-Up Right coordinate system, the Y coordinate in the window will actually be the inverted Z coordinate in Panda. This node is especially helpful when you want to do pixel-perfect positioning and scaling.
Panda3D includes support for easily rendering dynamic text onscreen or in the 3-d world. It supports full use of the Unicode character set, so it can easily render international languages (including Asian languages, when used with an appropriate font).
Rendering 3D Models
It is possible to render 3D models in the 2D scene graph. However, because the depth buffer is disabled by default on the 2D scene graph, self-overlapping models may not render correctly. It is necessary to turn on the depth buffer for these models. This is explained on the page Depth Test and Depth Write.