In short, ability to select between units of one category while adding entry, not when editing library, also ability to add custom category of units with your own units (for example different current units: micro amperes, mili amperes, amperes etc), with ability to convert between them (i.e. you want in your library a power unit, so you set watt as your unit, but you want some entries in your library to use kilowatt or megawatt, you can't do that with one field only, you would need three of them.)