How to understand -90kPa
This is a way of expressing vacuum in terms of relative pressure, the difference between some pressure and atmospheric pressure. Its value is between 0 and -101.325 kPa.
For example, we often see advertisements for vacuum cleaners such as "high vacuum 9000 pa", here 9000 pa, actually refers to negative 9 kPa, that is, absolute pressure 101-9 = 92 kPa, low vacuum in low vacuum, far from high vacuum.
Similarly, negative 90 kPa means the absolute pressure is 101 minus 90=11 kPa.