That is defined in NATO article 5. "
The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations,
will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force"
In more simple terms:
Parties in NATO are supposed to reach peaceful means to conflict resolutions.
But, if there is an:
(1) Armed attack
(2) Against a NATO member in Europe or North America
then they will assist the attacked parties with actions
(3) as deemed necessary
(4) including the use of armed force.
I think that which ones can and which ones can't is defined. See (1) and (2) above. Otherwise, the NATO members are supposed to find peaceful resolutions. Yes, NATO could clarify further exact details and give good examples. But, that is where things are now.
NATO made a very open ended (too open ended in my mind) decision that there was an armed attack against a NATO member. During negotiations on that first day, they did go back and insert an "if" into their statement. That "if" statement narrowed their decision down quite a bit. But it is still exceedingly broad.
Edgar Buckley describes how NATO invoked Article 5 on 12 September 2001, 24 hours after the terrorist attacks against the United States.
www.nato.int