This figure illustrates a rollup for a dimension with two hierarchies. In it:
year: is at the top of the dimension hierarchy
quarter: is below year
month: is below quarter
day: is at the bottom of the dimension hierarchy
In addition, there is a second rollup. In it:
fis_year: is at the top of the dimension hierarchy
fis_quarter: is below fis_year
fis_month: is below fis_quarter
fis_week: is below fis_month
day: is at the bottom of the dimension hierarchy
Therefore, day rolls up both hierarchies.