In real-life scenarios, it is quite common that we don’t know which resources exactly would be available when the activity is ready for execution. It might depend on work priorities, resources availability, their productivity and cost.

But what we know most of the time is that what kind of resource skill would be needed for the required activity. 

Resources belong to the same skill set if they can do the same type of work. They might have different costs and productivities but can still replace one another to get the work done.

Another term used for skills is a pool of resources.

Skills are usually assigned when it is not clear what resources will be available at the moment when an activity becomes ready for execution.

A resource may have more than one skill and may belong to different pools.

At any moment when there are changes in skills pool participants, these changes can be easily applied to all future skills assignments.

See how Resource Skills Assignments work in Spider Project: