Robustness can be split into three major categories. Mechanics, Knowledge and Perception.
Mechanics is how well you can function in combat, dodging, shooting and smacking.
Knowledge is your ability to know what item is more valuable, good hostage positions, how many hits you can take with certain armor.
And, as a foot note of minor importance is Perception. Basically your ability to know when to apply your knowledge and mechanics for optimal effect.
You can not declare yourself robust. It is a title of respect given by others to acknowledge 'hey this go is pritty gud'