Undercall multiway

[Guillaume Gleize]

Hello!
Did I understand correctly the UNDERCALL with no verbal declaration:

1) Check + Open bet + Fold + Call + Undercall > THE UNDERCALL MUST BE COMPLETED 
(because here we only face the open bet, even after the first call that changes nothing right?)

2) Check + Open bet + Fold + Raise + Undercall > THE UNDERCALL CAN BE COMPLETED OR FORFEIT
(because here we don't face only the open bet but multiple bets right?)

3) Big Blind + Fold + Raise + Undercall > THE UNDERCALL CAN BE COMPLETED OR FORFEIT
(because the Big Blind is actually an Open bet right?)

Conclusion (if I'm right) : The most numerous cases oblige to complete the Undercall.
Finally only the raises can give the choice to forfeit the Undercall.

Tell me ...