If I understand correctly, "Planned For" is intended to be a project's timeline iterations.  So if I'm using a fairly out of the box configured project, I have a timeline and iteration breakdown that looks something like this.

Release 1.0

...Requirements

...Design

...Implementation

...Testing

When I change the "Planned For" on one of my work items, the history shows this:

Are "Release 1.0/Testing" and "Release 1.0" both storred in the same field?  Is there any distinction between them?  How can I access the distinction?

The reason I ask is I would like to be able to define an iteration breakdown under Release 1.0 but query some field to identify which release a work item it marked for.  In other words, even if a work item is Planned for "Testing", I want to be able to run a query that will tell me that the work item is designated for "Release 1.0" regardless of the specific iteration breakdown the work item is assigned to.

Also, is there any functional relationship between the Planned For and Release attributes?

Ultimately I need to query this field via the RTC work item reportable REST API.  I am currently getting the Planned For value from the REST API target/name field.  But I'm willing to change to a different field to get this information.