Lennox in all there great wisdom decided to rate there cond. units in heat rejected. ie 1 1/2 ton pickup 18000 btu reject 18000 + 20 - 25% you get 21000, the third # for phase. 1 1/2 ton 3 phase 213. I have numbers but just add the 20 - 25% for size.
The HS18 was a "builder's special" model - around 8 or 9 SEER.
I haven't worked on enough Lennox units to know that (they aren't popular here) , but by golly- you helped me out too. I called Lennox once to find out what the #'s meant and he told me in as to say "DUH!"