Talking before this; hey what's our plan if Russia invades? Ok, what about if Russia does a "small" invasion? Etc. "Informal" in that they're not producing anything that requires a vote or action from member parties - it's meeting of established joint committees, distribution of memoranda, back and forth on policy.

Talking now: which of the plans we have discussed before are we going to enact? Formal meeting, in that a member state has declared it's intent to bring a topic to the table and that the body is obligated to discuss and determine a course of action - which is allowed to be "no action".

Basically, before now everyone figured out what to do on paper, now the Treaty Organization has to make a decision about next steps.