how is my <prodname> element being mapped to a variable named "BM_Prodname"

I have inherited a FrameMaker project that is missing some documentation. I am trying to understand how something was accomplished (so that I can modify it...).

In our .DITAMAP files, we have <prodname> elements. Somehow, when our PDF is published the contents of the <prodname> element appear on the Title page of our PDF.  This is great, but I am trying to understand how that mapping occurs.

Setup:

• FM2020 or FM2022 (works on both)
• We are using DITA 1.3.
• When I publish a PDF, we use a .STS file:
  • on the DITA Options tab it has Apply Output Templates checked.
  • On the DITA Template tab, I have previously imported from TitlePageTpl.fm
• When I open the TitlePageTpl.fm file, I can see a variable has been inserted in the exact location where the <prodname> contents magically appear in the PDF output (i.e. clearly this is somehow "inserting" the correct title":
  • The Name of this variable is BM_ProdName
  • The Definition of this variable is Product

What I cannot see/figure out is how <prodname> is being mapped to the variable BM_ProdName...