And how do you define a matter of commandment for this purpose?
For example, in the responsa Hit’orerut Teshuvah (cited in Yalkut Yosef, Honoring Father and Mother, in a note), he wrote that if someone says he will do something for the health of his body, that too is considered a commandment matter
and requires annulment.
Also, what is the definition of a decision? For example, if you said to your wife, “I’m going to the study hall,” and afterward regretted it, do you now need annulment?

(By the way, this is what Rabbi Yitzhak Shmuel Shechter told me [if you know him, and it’s worth knowing him]: there are doubts and double doubts here, so they rely on the stipulation.)

The question is not what counts as a commandment, but what counts as a decision. For example, if you told your wife that you’re going to the study hall, that is a statement reporting something to someone, not a declaration of decision/vow. In that case, the decision, if there was one at all, was in your heart.

I don’t understand how it is possible to say about certain things that they are a vow.
Even if I said that I will do something, and even if it is a commandment matter, and even if I did it 3 times, and even if I intended to obligate myself, as long as I did not intend a vow-obligation,
I just can’t understand how one can say that this is a vow.

In my opinion, you can obligate yourself, and then maybe there is some moral obligation here toward yourself, but I don’t see how one can say that this is a vow; it sounds absurd.

Good question. Apparently the concern is that when a person obligates himself, he himself identifies it with a vow. Maybe this stems from a lack of awareness of the difference between Jewish law and similar non-halakhic acts (such as morality or an ordinary human commitment). It is also possible that some people specifically do not distinguish between a commitment and a vow. In their eyes, a vow is simply the halakhic way to obligate oneself.