The search models in Enterprise Search can be extended to search additional related attributes to enhance the value of the tool for business users. This blog will explain the steps required to accomplish this task by demonstrating on the Maintenance Notification model, BUS2038.
The business requirement behind this example was that users needed to be able to find notifications by searching for specific attributes on the associated equipment record or maintenance order. For example, to find all notifications associated to a specific equipment model number. While the equipment number is included in the standard Maintenance Notification model, additional equipment attributes such as model number are not included.
The users needed to be able to find notifications by searching all information shown in the diagram below but were limited to only the items inside the red box:

The model was extended to include all attributes in the diagram.
Step 1: Create Customer-Specific Software Component
Open the modeler in the Connector Administration Cockpit (ESH_COCKPIT). Select Software Components then select Maintain Software Components:

Input a name for the Software Component and Package, do not select any item in the Copy from SW Comp drop down. The name I chose was Z_EXTENDED, the Package is $TMP as this is being demonstrated in a test client:

Step 2: Include SAP Software Components in the Customer-Specific Software Component
Select the newly created Software Component then select Software Components and then select Include Software Component:

The EA_APPL Software Component is included as the Maintenance Notification search model is contained inside the EA_APPL Software Component.

Step 3: Create Association to Additional Models
Locate the desired model in the Software Component and then select Edit. The Maintenance Notification model, BUS2038, is found in:
Z_EXTENDED -> EA-APPL -> SAP_APPL -> [PM]

Navigate to the Node Relations section. The models for Maintenance Orders (BUS2007) and Equipment (EQUI) will be associated to the Maintenance Notification header. The association is added by selecting the top MAINT_NOTIFICATION Node then select Create then select Association:

Find the models and nodes required and then Select to continue:

Step 4: Add Attributes to Model
Once the association to the additional models has been added, the attributes from these models are available for selection in the Model Requests section. Navigate to the Model Requests section and then select Add then select Attributes/Groups from Node:

Select the required attributes from the relevant nodes and then use the Select button to continue:

Select the Freestyle option for all the attributes for which you want to be included in the Freestyle search:

Save and Finish Changes:

Step 5: Update and Index Extended Connector
Return to the Connector Administration Cockpit. The Connector will need to be removed and added again the first time after it has been extended, otherwise the standard Connector may still be active. If modifying an already extended Connector, select the Connector then select Actions then select Update:

Once the update is complete, the status will change from Preparing to Prepared. Once the status is Prepared, Schedule Indexing by selecting the Connector then Actions then select Schedule Indexing:

Select all models and enter the desired index settings. Ensure to select Clear Index Context in the Full Indexing Mode:

Once indexing is completed, the attributes added from the associated search models (Equipment records and Maintenance Orders) will be available to use to search for records in the enhanced model (Maintenance Notifications).