There’s two ways to advance Steel: tests as generated by combat or other circumstances where the degree of Hesitation matters; and nerve-affecting situations as determined by the GM and laid out in the examples.
When making a test, Hesitation gives you the base Ob, and is modified by the Advantages and Disadvantages for the circumstances. For instance, “being startled by something mundane” gives a +2D advantage, which will lower the relative difficulty of the test. “Committing Murder” gives +2 Ob and will increase the test’s difficult accordingly; if it’s your own mother you’re killing, I’d imagine an even higher penalty is in order!
The circumstance examples at the end of the chapter give alternate Obs to use for the purpose of logging tests for things that happen in session but aren’t rolled for. Witnessing a friend being killed clearly tests your resolve, but it might be that the situation in game didn’t require to know how long you sit there unable to react to the event. If that’s the case, you bypass the roll but log an Ob 4 test to your Steel at the end of the session