Hello,
The available IP in the pool (say p.q.r.s) is chosen for translation for internal IP say a.b.c.d when it tries first connection out (say telnet).
Any different flows from the same IP a.b.c.d (say ssh or http) will use p.q.r.s
This binding will continue as long as you have at least one active session where translation from a.b.c.d to translated IP p.q.r.s is present.
If you have a situation where there is no active session using a.b.c.d to p.q.r.s translation, then the binding will be over.
Regards,
Rushi