Well all curses have a danger level, hence, the grade system. Machines do an analysis on the curse and make up it's power from that info.
"The seven types of grades are very basic. First we have grade 5 curses capable of destroying a house or two, nothing more. Then comes grade 4, who can cause street level destruction, around a small apartment complex. Grade 3 curses, capable of tower level destruction. Then the grade 2 curses, capable of levelling small neighborhoods and with the grade 1 curses being able to destroy small cities. The next two, however, are special.. Those two being semi special grade curses and special grade curses, with the semi special grades being able to level Tokyo, and the special grades, though only 3 exist, can cause mass destruction which hasn't been measured in all of history yet. Even so, with your powers, you would be able to defeat them easily, i wouldn't worry about them, at least not while Sukura Goen is still alive." All this being explained by Mister Katsu, Kuroto's sensei.
"So then, which grade would the curses i fought in the ambush be? I assume grade 4 and 3, they were quite disappointing after all." Kuroto replied, to which he got the answer "Well, you already got the basics pretty fast, yes they were."
With all that said, Kuroto's knowledge session seemed to be over for the day. While heading back home, he noticed a good spirit fighting a curse, seeming to be grade 4 from his analysis. He watched how the good spirit summoned creatures from out of nowhere and easily slaughtered the curse. The good spirit noticed him, and said, "Hey.. your Kuroto Ryuzen right?" to which he quickly responded "Yes indeed, and i see your a cursed ikimono user.. if you keep that pace up and increase the output of your technique, semi special grade curses wouldn't be a problem. Keep up with it."
Kuroto made sure to reach home early after that, getting a good rest. Tomorrow was his first day of Spirit Techniques school, a school made specifically for good spirits.