Application Support folders
Application Support folders are special locations where programs can store important files. OmniGraffle checks these folders for Diagram styles, stencils, templates, and AppleScripts. Each of these file types has its own appropriately named folder inside the OmniGraffle Application Support folders.
Files in ~/Library/Application Support/OmniGraffle/ (where ~ is your home folder) are available only to your user account. You don't need any special privileges to add or remove files there; it's your own special space.
Files in /Library/Application Support/OmniGraffle/ are available to any account on your Mac. You need administrative privileges on the computer to add or remove files there.