"Units" represent an arbitrary and substance specific agreement by scientists on activity level of a particular substance. You have to look up the agreement. "IU" is international units, and some worldwide scientific body has agreed on the definition for a particular substance. Without the "I" it must be a national agreement, in the case of no international agreement, or disagreement with the international agreement. As a minimum, you would have to know what nation and what scientific body within it defined the unit, and why there is no (or a different) international unit.