A pane hierarchy must be attached to a display server so as to permit input and output. This is handled by the use of ports and grafts . A port specifies the device acting as the display server, whereas grafts are special sheets, typically representing the root window, which are directly connected to the display server. (The term graft is derived from the horticultural practice of grafting, in which the trunk of one tree is joined onto the rootstock of another.) Again, a CLIM application programmer will not normally deal with these objects directly. A call to make-application-frame automatically results in a port specification and graft instantiation. Refer to 9.2, Defining CLIM Application Frames for details.
Common Lisp Interface Manager 2.0 User's Guide - 20 Sep 2011