I thought the answer would be in Pub 17 but it refers you to Pub 550.
Liquidating distributions, sometimes called liquidating dividends, are distributions you receive
during a partial or complete liquidation of a corporation. These distributions are, at least in
part, one form of a return of capital. They may
be paid in one or more installments. You will receive Form 1099-DIV from the corporation
showing you the amount of the liquidating distribution in box 8 or 9.
Any liquidating distribution you receive is not
taxable to you until you have recovered the basis of your stock. After the basis of your stock
has been reduced to zero, you must report the
liquidating distribution as a capital gain.
Whether you report the gain as a long-term or
short-term capital gain depends on how long
you have held the stock
I think since I wrote that last reply, I found out some more about this subject. I had a couple clients who received substantial CLD's from the same stock -- Altaba. Funny thing is, they also received them in 2019, and I'm not sure I reported them correctly. Turns out Altaba is what Yahoo turned into, after all they had left was some Alibaba stock that the predecessor AOL's CEO had thought might be a good idea. (I might have this history wrong.) Anyway, most investors probably recovered their basis, and then some, in 2019; by 2020 it was all LTCG. On Schwab 1099's, no explanation was given. On Merrill Lynch 1099's, the proceeds also showed up on the 1099-B sales reports, with basis (at least for one client) shown as zero.