Process Message 
Definition
Communication structure that is used to transfer actual process data from process control to one or several destinations of the following types:
Other SAP application components
User-defined ABAP tables
Users of the SAPoffice mail system:
External function modules
See also: Process Message Destinations
The information contained in a process message is determined by the message characteristicsas well as the characteristic values assigned to them.
Use
Depending on the destination to which the process message is sent, you can use the Process message business object for the following:
Triggering business transactions in the SAP System (for example, goods issue posting)
Documenting actual data from process control and evaluating this data using internal or external functions
Sending mails to certain employees and informing them about the process status
Sending actual data from process control to an external system and triggering system reactions (such as opening a valve), if required
If process control is carried out by an external system, too, the actual data must:
First be sent to the SAP application component PP-PI (process message upload) via the PI-PCS Interface
Then be transferred to an external system from PP-PI (process message download) via the same interface
Depending on the process instruction category , you can define one or more process messages in a process instruction by assigning one ore more message categories along with the message characteristics to the process instruction during order planning. You can do so as follows:
In the process instruction categories in Customizing for Process Management by choosing Process Instructions -> Define Process Instruction Categories (General)
In the process instructions in the master recipe and the process order
Structure
A process message consists of a message header and several message characteristics as well as the characteristic values assigned to these characteristics
Header
The header of a process message contains general message data such as:
Administrative data on the message
The plant in which the message was created
The message category to which the message belongs
An indicator that specifies whether the message was created for test purposes or whether it should actually be sent
Characteristics
Which process data is to be contained in messages of a particular category is defined by assigning characteristics.
Process message characteristics are defined and assigned to the message categories in Customizing. In the message category, you also specify whether a characteristic is required or optional.
See Customizing for Process Management:
Process Messages -> Process Message Characteristics 
Define Characteristics for Process Messages 
- Process Messages Define and Set Up Process Message Categories
Process management checks all incoming messages against the predefined message categories. Each message must contain at least those characteristics marked as required within the message category.
When you create or change a process message manually within SAP System, the characteristics assigned to the respective message category are automatically offered for maintenance. You cannot add further characteristics to the message.
Characteristic Values
In the process message, a characteristic value can be assigned to each characteristic contained in the message category. The value expresses the process-specific data for the characteristics.
The format of a characteristic value as well as the value range to which it must belong is defined in the characteristic in Customizing. Depending on the format, the value of a message characteristic can be either a date, a time, a numeric or alphanumeric key, a short text or a long text.