The Context Menu
Unlike the Spawn Menu, the context menu is, as the name indicates, a context related menu.
Except for the top bar it is mainly used to manipulate parameters of in-game objects.
You can also aim weapons with it without changing your aim angles.
Open the context menu by simply pressing and holding c or whatever button you have assigned in the Game Options. You will notice that you no longer control the crosshair, but rather a mouse cursor. Choose the object you want to edit by pointing at it and then click right.
The top bar is always present and displays 2 options by default.
The drawing menu allows you to customize what will be drawn on your screen.
The NPCs menu allows you to alter NPC behavior, and force spawned NPCs to have specific weapon.
Desktop Widgets are also always visible and give you one option by default.
The player model widget opens a window that allows you to customize your player model.
This includes choosing your player model, it's color if the model supports that and also your Physics Gun color.
Please note that you are only selected your "preferred" model here, multiplayer servers may override your player model.
If you are using a model from an addon and server doesn't have that addon, you will appear to everybody as Dr. Kleiner, the default player model.
By right clicking most objects you will open a menu that contains certain options relevant to the object you chose.
For example, an NPC's context options will look like this: