To sum up, to build "cascade select in form", please refer to pier-olivier's solution.
It seems there is no safe way to achieve "cascade select in table" in SC 17.1
Although we can use javascript freely to manipulate html elements in DOM level, I think it is not standard customization way which is hard to maintain and risky for version upgrade.
So we will reject this requirement to the end user.
As a workaround, we provide below table to combine all the "cascade select dropdowns" into one dropdown.
Category-Application-Function column
----------------------------------------------------
Category1-Application1-Function1
Category1-Application1-Function2
Category1-Application2-FunctionC
Category1-Application2-FunctionD
Category2-Application4-FunctionY
Category2-Application4-FunctionZ
In this single dropdown, user can type the keyword to search. So the selection time per row is actually faster than selecting three dropdowns one-by-one.