It refers to excess duty periods available in ELITT.
Practically, it means being able to trade down in ELITT. For example, if NetZero is 1, that means you can trade your 4-day down to a 3-day in ELITT.
The way NetZero goes up is when a pilot trades up, say from a 2-day to a 3-day. For example, in MCO, they have some weird stuff in the water there so their NetZero is through the roof even in the summertime...
Whereas, in the other domiciles, at least in the summertime you're lucky to catch it be 1 so you can occasionally dump that 4-day for a 3-day or a 3-day to a 2-day, etc. In the winter time, that NetZero number is a bit looser everywhere in the system allowing a bit more ease to drop trips, except in MCO where it's on crack and ELITT gets cleaned out with guys picking up extra periods driving NetZero really high.