1.If there are no objects selected,this will random select the objects can be seen in active viewport according to the percent.
2.If there are more than 1 objects selected,this will random select the objects in selection according to the percent.
3.If there is one EPoly selected and it's under EPoly modifier,this will random select the subobjects which is active according to the percent.
4.You can directly random select subobjects by right click this button,attention that the object is under EPoly modifier.
5.Under the border subobject level of EPoly,this will select all of the open edges.