How to populate a node reference field based on the selected taxonomy term?
I would accomplish this by arranging your vocabulary like so:


Category 1

Task 1


Task 2

Category 2

Etc.



I would probably just name it Service Tasks, but that's my preference. Then use the hierarchical select module.

Enable the module and it's Taxonomy Select sub-module, then add a term reference to Services using the hierarchical select widget. Now you can choose tasks related to their parent categories without having to make all sorts of references.