What is the difference between ius and mgs next to vitamins..?

what is the difference with mgs and ius next to vitamin D.What is the difference between ius and mgs next to vitamins..?
The recommended dose for an adult is 400 IU per daylight which is easier to remember than the same degree expressed in milligrams.IU or international unit refer to the phisiologigal action of the vitamin while the mgs refer to mass.
Basically mgs is a width of volume while IUs is a measure of the effect. Its more complicated than that but to be precise an idea of the starting place.