two usages of continuations

[Matt Wette]

Hi All,
I'm currently reading a paper that references [1] to state that there
exist two usages of continuations.
1) if invoked continuation is prefix of current continuation, it is
downward
2) otherwise it is upward
Would anyone like a shot at elaborating this?
Does this mean a called continuation is downward if it is a tail call?

I found more on this.  A downward invocation is an escape.