[chucksmash]

A lot of the apparent ambiguity could be resolved by looking at the number of launches and their trajectories:

A) there are simultaneous launches against many different targets. Conventional or not, the enemy is trying to remove your ability to retaliate. The implication is clear.

B) Only a small number of weapons have been fired off at one or just a few targets. Even if the weapons inflict heavy casualties, your ability to retaliate remains intact.

[mikeash]

There's another option as well: have enough redundancy to have substantial forces survive even a massive first strike, absorb it, and then retaliate at leisure. This seems to be the US's approach:

https://www.armscontrol.org/act/1997_11-12/pdd

I imagine other countries with smaller arsenals may not have that option. Even your B may not necessarily be an option, if their command structure relies on centralized control and would be vulnerable to a decapitation strike.