Sims 2 Neighbor ID
This a number which each Sim has to represent it, whether the Sim is instantiated on the Current Lot or not.
It is different from the Object ID, which only those Sims on the current lot will have.
It doesn't change if the Sim moves out of one lot and into another - the Object ID does change.
A Sims Neighbor ID can be found in their Person Data.