It is not the unit ID. In one of the the unit data files (I think it's Units\UnitsUI.slk), you can find units by their unitID in the leftmost column, like 'nomg' for Neutral Ogre Magi. Each unit then has many properties in subsequent columns, like file (the path for the model file), name (the name for the unit displayed ingame), and unitSound (the path for the voice file). Some of them are somewhat obscure, like useClickHelper (no clue), but most of them do have some function if you look deep enough.
Some values though, like mana0 or realdef (in a different spreadsheet, maybe UnitData.slk), appear literally useless. One of these is unitClass. But every unit in WC3 is given a class, like 'HHero03' (presumably for Human Hero 3) or 'animal' or whatever. It seems like it should mean something. Just wondering if it does.
E: Confused 'name' with 'Name'. Apparently the lowercase one is not the name used ingame. Not sure what that one's for either.
Blizzard is weird.