Visiting other Schengen countries using a Schengen residence permit - seeking clarification of the exact meaning of "90 days per 180 day period"

Question

A lot of questions were asked regarding people outside of the Schengen area visiting the area using the 90/180 rule. However, I am curious about the following scenario: One has a residence permit in a Schengen area country A, but he may want to stay in another Schengen area country B for an extended period of time in a year. According to info from this page,

If your long-stay visa or residence permit has been issued by a Schengen area country, you can travel to another Schengen area country for 90 days per 180 day period.

This means he can only stay for a maximum of 90 days in country B during these 180 days, which seems clear. However, does it mean that he'll then theoretically be unable to travel to any other Schengen country during this 180-day period, and will have to spend the other 90 days strictly in country A? I find the wording not so straightforward regarding this point. (Or maybe I just didn't understand the sentence in full.)

(I know that from e.g. this question there doesn't seem to be any strict checking of how many days you actually stayed in another Schengen country anyways, as long as you didn't leave the bloc. But regardless of how it plays out in practice, I'm wondering about the theoretical aspect of the law itself.)

EDIT: I just realized that, even if the above interpretation is correct, the person might not need to "strictly spend the other 90 days in country A" after all. He can actually spend some days outside of the EU altogether after his stay in country B, until he returns to country A within 6 months, so that his residence permit doesn't get invalidated (e.g. in the case of Germany), and this will still be legal. Does this understanding make sense?

Answer 1

They may visit other Schengen area countries (outside of country A where they have a residence permit) up to 90 days per 180 day period

• countries B, C, D etc.

The days in country A or outside the Schengen Area do not count towards the 90 days.

The 180 day period moves foward one day daily (independent from where you are).

Today, if you were in country B 180 days ago, that day would count towards the 90 days.

Tomorrow that day, 181 days previously, would not (it has 'expired' because the 180 day period has moved foward one day).

On a daily basis, you must count how many days you were in other Schengen countries.

If more than 90 days, then you must remain in country A (or outside the Schengen Area).