I think it is different for everyone

Here are some examples, but highly colored by my opinions:

No Boundary=T lets you call anytime whenever you want
Flexible boundary=you can call occasionally or when in crisis
Hard Boundary=no contact between sessions

No Boundary=unsolicited hugs from T without asking first
Flexible boundary=you can occasionally ask T for a hug
Hard Boundary=No touch, no handshakes whatsoever

those are 2 examples, I guess everyone defines it differently