Process Message Category 
Definition
Process message categories are standard process messages that have been defined in Customizing for Process Management .
The standard system comprises a number of standard process message categories that you can use or copy to define your own process message categories. You can create your own process messages in Customizing for Production Planning - Process Industries by choosing Process Management -> Process Messages -> Define and Set Up Process Message Categories.
See also: Integration Using Process Messages
Use
Message categories are defined per plant when you customize your process management system.
The process message category specifies:
Which message characteristics are to be contained, which means which information is to be passed on in a process message of a certain category
The message destinations to which the process message is to be sent
If required, the destination-specific target fields to which the individual message data is to be transferred
This offers the following advantages:
The structure and destination of similar messages need to be defined only once in a plant.
Only the users defining message categories in Customizing must have a detailed knowledge of the destinations and the data they need.
The persons who create process messages during the production process can use the predefined message categories.
Each process message sent via process management must belong to a predefined message category. You need process message categories:
If you want to define process messages in a process instruction category in Customizing for Process Management under Define Process Instruction Categories (General)
If you want to define process messages in the process instructions of master recipes and process orders
If you want to create process messages manually in process management