Reference https://docs.oracle.com/cloud/r13_update17b/projectcs_gs/OAPJC/OAPJC1071490.htm#OAPJC1071482

Reference https://docs.oracle.com/cd/E37583_01/doc.1116/e22598/F458900AN13D82.htm How Resource Mapping Is Calculated You can track the cost impact of every resource that has been assigned to a project task and use the resource breakdown structure to view the breakdown of these costs. Oracle Fusion Projects associates the costs of the resources used for tasks with branches and levels in the resource breakdown structure. The process for determining the correct association is managed by rules of precedence.
Oracle Fusion Projects uses the following rules to associate cost amounts with resources:
Select the lowest level in the resource breakdown structure to which a transaction can map.
If there is only one level to which the transaction maps, the cost amounts are mapped to that level.
If the transaction maps to more than one level, Oracle Fusion Projects sums the precedence numbers for all resource types in the branch and gives precedence to the resource element in the branch with the lowest sum.
If more than one branch has the lowest precedence number at the lower level, the application uses the precedence number of the next level up.
If the sum of precedence numbers is the same for more than one branch, precedence is given to the branch with the lowest number at the lowest level.
If one branch contains a user-defined resource type, precedence is given to the branch that does not contain a user-defined resource type.
Note Oracle Fusion Projects gives more precedence to a lower precedence number. For example, a resource element with a precedence number of 1 is given precedence over a resource element with a precedence number of 10.