The ballast.
Different filament characteristics and possibly different gas mixtures.Are there two specific t5 bulb designs...one HO and one NO...thanks!
Are you referring to different bulb brands not fitting specific fixtures or ballasts? Or did you mean that "our" refers to the U.S. vs. other countries?There are also, different lengths of T5NO lamps. Some of them have different enough electrical characteristics to not run properly on "our" standard T5NO ballasts.