As you said, in the decomposition rule we use & need only fromOrderComponets. We wouldn't need to refer toOrderComponets(after all they will become on order component after all decomposition stages ie., SyncCustomerFunction[BRM-ALL][OrderGranularity]) in decomposition rule but the same toOrderComponets is very much required in setting dependency(ie., in ProductSpec).
In the Design Studio Help, please refer to "Working with Decomposition Rules" inorder to understand Decomposition Rules.
Property Correlation is used while setting a dependency in the Product Specification. This is where FromOrderComponent & ToOrderComponent both are extensively used. For more information, please refer help section for the following.
1)About Order Item Dependencies
2)Orchestration Dependency Editor Order Item Dependencies Tab