This item is very arcane and until recently just another little mystery to me. While working on the egress family and its tag family that I made and posted about in the past I noticed the following item.
I hadn't noticed it in the past and searched for any information that might be available in the help files. Truthfully I wasn't too surprised when nothing jumped out at me in a few tries at a search. Unfortunately the help files are bit too sparse when it comes to the family editor "secrets".
So I posted a question to the gallant folks at Autodesk and I received this explanation:
“It is used to control which elements are taggable by a multi-category tag family. You can select a shared parameter as the filter parameter, and then when placing a multi-category tag, the tag placement tool will only let you place the tag on elements that contain that parameter (either because the parameter was added to the element’s family or because it was added to the element’s category as a project parameter).”
(My thanks to Steven Campbell and the "unknown" developer who came to my aid!)
This applies to Multi-Category tags. As it applies to my Egress family, this means I can tell Revit to filter for a specific parameter so that my "Multi-Category" tag will only tag "my" family..or any other that I associate that parameter with.