it's a seal actually. Under normal circumstances in the book, the seal is important to keep the demon within at bay
Verbal seal, if the seal was meant to keep the "demon at bay" <-- I am guessing you mean Frankenstein powers, than it would make all sense. But it really isn't that type of seal. The seal came about because Frankenstein used to experiment on people with his powers, to avoid those experiments Rai placed it on a "Seal."
For me, An actual seal would mean that Frankenstein wouldn't be able to access it without Rai permission. Also Frankenstein powers doesn't run wild on his own without the seal, more like when angered or in a desperate battle does his powers turn crazy. Before the seal, he always avoided his weapon from taking control, meaning that he could control his powers by himself without the need of the seal.
Not the story, but the whole restriction of powers is pointless. Let's be honest, the power restriction is a command not a real seal that was placed on his abilities. He could use the full extent of his powers without his masters command, he's been able to break it as much as he wanted to. The only difference is that Rai could notice it if he breaks it. Plus, doesn't Rai notice that Frankenstein is all his, their is no need to be restrictive since his servant isn't going to go crazy and experiment on people like he did in the past. He's 100% loyal, so he could run without the seal. I feel like the verbal seal is a way to ensure he remains loyal, like Rai doesn't fully trust him. I guess it just me who looks at it like that...
Anyways i hope theirs more to this verbal seal, like maybe the verbal seal actually blocks those powers that are said to be equal to that of a noble. If the seal is meant for that than i would be 100% happy.