Use Get Referenced Item(s) to collect the data from any Item that is referenced to the current Item using Podio's Relationship Field. This collector is not required to update a referenced item.
Use the Get Referenced Items collector when you require data from a Relationship item to use in your actions. Be sure to set up the referenced Direction and Using Field in order to correctly identify the items required in the collector.
Use the Reference Map to correctly identify your relationships and directions. These Reference Maps can be accessed from the Organization or Workspace Level. To view this image larger, click the Image link above the map.
Black Arrows are a FORWARD relationship meaning the relationship field is on the trigger item
White Arrows are REVERSE meaning the relationship field is on the destination item.
Note: App Items can only be collected ONCE. In order to collect different items from the same app, the previous items must be cleared using a Clear Collected Item(s) action first.
Example
Start the flow using the appropriate Trigger on App A
Add Filters for required conditions
Then add the Get Referenced Item(s) to collect the fields from the Customers App B.
Note: Set the Direction to open up the Using field and select your Relationship field. If there are no options in this drop-down menu, your collector has not been set up right. This can be adjusted by changing the Direction.
Add in Actions required to complete your flow.
The Referenced (Ref) Item Fields will be available in the token selector. 
Troubleshooting
If your flow does not seem to be pulling in the correct data, be sure to set the "Using Field" to your Relationship field in App A which references the Items in App B. Using BOTH Directions and the Any Field may cause confusion in following the reference to the items you require.
Note: The Get Referenced action only returns 1000 (max limit) related/referenced items. If the collected item is referenced/related by over 1000 related items, the first 1000 related/referenced items will be collected and used in the remaining actions.
See this feature used with multiple Relationship fields to the SAME App in our Lab: Collecting multiple Relationships Fields