It changes.
Fans almost never run 100%, most times getting a case fan naturally past @ 70% is a chore and you can not go by cfm as a basis as that changes too, all you see is max rpm values, which are pretty meaningless. Fan cfm does not respond linearly. In other words you might get 100% cfm at 100% speed, but you will not get 50% cfm at 50% speeds. Depending on fan blade design, rpm etc you could get anywhere from 20% cfm to 90% cfm at 50% speeds.
So it's entirely possible to have a negative at idle, neutral at gaming and only get a positive at max, or the other way around.
And that's only in a sealed system. There's generally enough unused fan vents, gaps in slots by the gpu, mesh next to the exhaust fan etc that negative/positive is an attitude more than a reality.
When a fan blade moves through air, it creates a low pressure area behind it. Get enough blades moving through that area, at high enough rpm, you create a low pressure area that extends into the case. Nature abhors a vacuum, so the air inside the case will move to fill that low pressure area in a constant flow. But, that low pressure area will be filled by the nearest available air, if it's an empty top mount fan hole, that's where the air will come from, easy access. A negative system is where there's not enough supplied air to adequately fill that low pressure area and the vacuum is using outside air next to it. A positive system has enough supplied air that it floods the low pressure area, not allowing for much (if any) outside air draw.
At idle speeds, you'll more than likely have a negative system as there's simply not enough input air to flood even a relatively weak exhaust, it'll pull the air right next to it first. At max rpms that'll change. Generally.
One of the best airflow cases available is the fractal design meshify c. 3x 120mm intake, 1x 120mm exhaust and relatively well sealed, no other fan vents. This allows the vast majority of exhaust draw to come from only 1 place, the case, which is well supplied. If the case had 2x 120mm top vents as well, airflow would go in the toilet as that'd be where the exhaust fan would be pulling all its air from first.