Pratyush,
As this question relates to Form Templates, it's important to distinguish the difference between data that appears on the ByDesign User Interface and data that appears on the Form Template.
On the User Interface, there's a variety of functions built in that are used to retrieve data from other areas of the system.
However, for Form Templates, you're restricted to the base XML that is used to generate the Form. Typically, the base XML contains all the fields that you'd see on the screen, but this may not always be the case.
My recommendation is to go into a Purchase Order that has been sent, and go to the Output History tab. You'll see a table with the output document (you'll need to click Refresh first). You'll see a button that says "Download XML data". Click that. It will download a .zip file. Extract the zip and you''ll see a "test.xml" file. Open this in an XML Editor (like Notepad++).
Within this, you'll see all the data that is available to be included on the Form. Look through this and see if you can find a value that looks appropriate. You'll need to check that Reporting Line Unit actually appears on the base XML if you want to use it. Otherwise, you'll need to use an Extension Field and populate it manually (or through customization)
For more information about adding additional fields to LiveCycle, take a look at this blog.