HIG means "Human Interface Guidelines".
csdi stands for controlled single document interface. The gimp which is the prime example of this interface uses a control window (the main toolbox window) as the control for other windows in the application. This interface is frowned upon and seth even tried convincing them to switch to a standard sdi interface but they weren't listening :/
In the bug seth states the following in reference to the gimp: "I tried to convince GIMP developers to do the same trick as GNOME does with toolbars with a menubar in each image window..toolboxes and pallettes can be shared between multiple windows without having the toolbox itself be the primary "controlling" window as in CSDI."
Basically the toolboxes become transient dialogs and each image window has a menubar which is used to perform actions upon the image (think of any document editor here). Typical menus are file, edit, view, and help, but feel free to use some sane judgement here.
-- David Adam Bordoley [bordoley at msu dot edu]