A foreign pointer.
A pathname designator or
A function designator or
An association list.
A string, or
A function designator, or
Otherwise, if filename is non-nil, it must be a pathname designator for a file containing DH parameters, which is loaded (by read-dhparams) and then used. In this case, pass-phrase and callback can be used, and are passed to pem-read.
Otherwise (that is, if each of dh, filename and func are
nil) then filename-list must be a non-nil association list of keylengths and filenames, sorted by the keylengths in ascending order (that is, larger keylengths are towards the end of the list).
set-ssl-ctx-dh installs a DH callback which when called finds the first keylength which is equal or bigger than the required keylength, loads the associated file (by calling read-dhparams), and returns it. It also loads the first file of the list immediately.
LispWorks User Guide and Reference Manual - 20 Sep 2017