Partitioning is the ability to split data into multiple strips.
When dealing with reactive sequences, partitioning means splitting a sourcing sequence into multiple subsequences. The goal may be to maintain message flow consistency when dealing with multiple parallel sequences. In this case, although we may prefer working with data from all the sourcing sequences in a single sequence chain for simplicity and maintainability, at a time, we will need to split the messages from their original flow by grouping them by one (or more) properties.
In reactive programming, we never partition for performance needs (such as parallelizing processing) because this is achieved by default using the reactive framework itself if we use the proper overall design (refer to Chapter 6, CLR Integration and Scheduling, for a more in-depth the argument).