IIUC it was a bit of both. A unit of work was meant to be short lived, but if a higher priority unit of work came along it would be pre-empted and its state pushed out to a temporary store.

There's a full description in the article, though it's buried a bit far down. Here's a snippet for grep-ability:

    if a low-priority job was executing and a high-priority job was scheduled, the low-priority job was suspended while the higher-priority job executed.