In Mail Merge 

• it supports {MERGEFIELD} & {DOCVARIABLE}
• it is better to use this format "ltr. serve order.doc" file name for your template document.

In Case Merge

• it support {DOCVARIABLE} only
• if the template document file name including "-BM" (i.e. Summons -BM.doc), it will auto switch to "BatchMasterFile" in "MasterFileSelector" when you selected it.