Global variables are more for Try usage, but basically it's Exactly What It Says On The Tin. If one part has an event that adds 1 to the first global variable, and a different part checks to see what the value of the first global variable is later, it will be 1. While you can basically do the same thing using the variables of the other parts in the boss, for Try scenarios this allows you to share data between bosses.

As for your second question, just make the proper parts invulnerable, give them an event that basically does "when I get hit, subtract health from part Y", and then halt that even once the parts are vulnerable again. No variables required.

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