The attribute end_state causes an order to leave a job chain at that state (more precisely: after being processed in that state).
If the order is set to the error state at the specified end state, it will not leave the job chain. Instead it will take the path that is configured for the error state.

e.g.

<job_chain name="test">
  <job_chain_node state="a" job="job_a"/>
  <job_chain_node state="b" job="job_b" next_state="c" error_state="error"/>
  <job_chain_node state="c" job="job_b" next_state="end" error_state="error"/>
  <job_chain_node state="end"/>
  <job_chain_node state="error" job="job_error" next_state="error_end" error_state="error_end"/>
  <job_chain_node state="error_end"/>
</job_chain>

<add_order id="order_a"/> will go: a->b->c->end
<add_order id="order_a" end_state="b"/> will go: a->b
if an error occurs in b, <add_order id="order_a" end_state="b"/> will go: a->b->error->error_end