autocomplete - What does the [?D] symbol mean in this context?

Question

On the documentation page for the tcsh complete command, it lists the symbols [?D] and [?C] several times. I haven't found anything from Google. Context suggests that they could be single or double tab, but I can find no reference to support this and wanted to make sure it wasn't something more esoteric.

Any ideas what these are meant to represent?

    > complete ftp 'p/1/$hostnames/'
    > set hostnames = (rtfm.mit.edu tesla.ee.cornell.edu)
    > ftp [?D]
    rtfm.mit.edu tesla.ee.cornell.edu
    > ftp [?C]
    > set hostnames = (rtfm.mit.edu tesla.ee.cornell.edu uunet.uu.net)
    > ftp [?D]
    rtfm.mit.edu tesla.ee.cornell.edu uunet.uu.net

question from:https://stackoverflow.com/questions/65916039/what-does-the-d-symbol-mean-in-this-context

Answer 1

It's a somewhat unusual way to write ^D, which is Control+D.

This is the "traditional" mapping to trigger completion, although Tab is more commonly used these days. Back in the day not all keyboards had a dedicated Tab key (the ADM-3A, on which csh was originally developed, didn't) and you would insert tabs with ^I, there is no real reason to prefer that over ^D in absence of a dedicated key.