However, I think in our own minds we can generally understand that a structure is some building or other construction that has walls and a roof. In California structures can include vehicles, such as railcars, airplanes, trucks, or ships. It doesn't have to be a permanent structure that sits only in one place, although obviously placing bait stations around an airplane that is going to move away would not seem to be in keeping with the spirit of the Rodenticide Mitigation Decision (RMD). We also need to avoid trying to work around this. For example, a previous question to this forum asked if a small bird house could be placed on some outlying fence and thus constitute a structure at that point, and my response was that this was highly unlikely to be accepted by any regulatory inspectors. 

Within the discussions on this issue from the rodenticide manufacturers, the information that we received from LiphaTech stated that EPA directs the states themselves to be the ones to interpret label wording. The manufacturers are not permitted to make these interpretations, so all they can do is add to their rodenticide labels exactly what EPA directs them to add, and how the wording is interpreted is then up to each state regulatory agency. On this information from LiphaTech they also state that EPA does NOT consider fences or fencelines to be structures. In fact, the actual wording now on rodenticide labels is "do not apply further than 50 feet from buildings" (not "structures", but "buildings"), so perhaps this is even more clear of a picture. The concrete walls around a dumpster would not conceivably be considered to be a building. A fence with 4 sides to it would not either. 

So, we will need to stick to the more likely definition that exterior bait stations must be placed within 50 feet of permanent structures that have walls and a roof, but do check with your own regulators to be certain of their intent.