A permalink is a hyperlink that points to a page whose content is in some sense permanent. Permalinks are mainly useful when one wishes to cite a particular version of a page. Like permafrost, however, a permalink may be less permanent than one might think. In particular, there is often a large gap between the source used to generate a page and the rendered version of that page.
On QED, a permalink is simply a reference to a specific version of the source of a page. The line in the navigation box labeled Permalink gives the "permalink" for the current page.
QED also has templates for flagging a link as being a permalink. For example, this DOI uses Template:DOI to display the DOI permalink. The rightward arrow in the red box is the flag; the superscript DOI indicates the link is an external DOI link. See also Template:permalink.
The entire URL of a QED permalink itself should not be regarded as a permanent reference, firstly because the server name might change, and secondly because QED reserves the right to delete content.
Apart from the servername component of the URL, however, the URL is invariant. To determine the invariant part of the URL of a particular version of a QED page, move the cursor to the button marked Permanent link in the toolbox box. A URL such as the following will be visible:
The invariant part (everything after the last '/') is shown in red.