You may wonder what does “Effort driven” option in Task Detail Form do?
Here I have 2 tasks.
Task A doesn’t have Effort driven check. When I assign 2 resources to it, the Work unit for each resource doesn’t change (80h for each resource). No matter how many resource I assign, each resource will get 80h.
Now, Task B have Effort driven check. When I assign resource to it, the Work unit for each resource change (with 2 resource assign it will get 40h for each resource). And activity duration reduce to 5 days. The more resource you assign, the less unit each resource will get.
SUMMARY
So, when you have kind of activity which can be reduced duration by adding more resource, use “Effort driven” option.
Khuong Do Blog 
You can choose activity type “fixed work” and get the same result.
LikeLike
Yes that’s correct. Fixed Work and Effort Driven mean the same thing in this context. In fact if you select Fixed Work, it automatically puts a tick in the effort driven check box and grey’s it out. :-). But you can have effort driven selected (or not) for Fixed duration and Fixed Unit tasks. In either of these cases the behaviour can be different.
LikeLike