1-year valid Visa C (short stay) with multiple entry - is 90/180 rule applicable?

One of my relatives got a visa C (short stay) and she is a non-EU person.
On the visa, it's written:

Duration: 90 days
And the validity period is 1 year, Multiple

What does this mean? In my first visit to the city hall, they told me that she can stay for 90 days during a 180-day period (known as the 90/180 rule). Then she must stay outside of Belgium for 3 months, then she can come back and stay for another 90 days, during another 180-day period.

In the second visit to the city hall, the previous person was absent. Her colleague told me that this is wrong, and my relative can only stay for a total of 90 days during 1 year.

Which is correct?