The Atomatic Resource Allocation routine assigns resources to activities based on

• Competences required by the activities and the competences  offered by the resources.
• Availability of resources.

The resource allocation routine respects all existing allocation, whether the have been created manually or by a former execution of the automatic resource allocation routine.

The algorithm used to allocate the resources is called 'Stable Marriage Matching'. 

See here how to perform automatic resource allocation.