We were taught in training that the old JATO (never got to fly the II) was a second segment climb cert. but
wasn't there for the cert. so it's relative to the instructor we had.

The 31K non-pax (non-oxygen) ceiling was a true ceiling in the sense that
it couldn't maintain 100fpm above it. (temp dependent)
But the plane was stable and maintained 31K with the conditions we flew it in.
To expedite getting up there (above 30K, climb was very slow) we'd fire water
from 30K to 31K (approx. 45 sec.) and then maintain a stable 31K without issue.