What is the procedure, or a URL to a Howto, for creating knowledge tags in FrameMaker 2015?
At what level are you considering tagging: document, page, topic? From what output are you going to be collecting the tags: PDF, HTML, or ... ?
At the document level, you can use the FileInfo option to add content.
The level is an entire technical document -- output will be to Adobe Acrobat (PDF). I have not tagged a document though I have constructed indexes.
How will you be aggregating the meta-data tags in the PDF? FM gives you a basic set of tags that you can fill in using the File > File Info... option.
You can add your own custom fields using postscript text frames and pdfmark /DOCINFO entries, but you need to work around the behaviour of the /DOCINFO when being distilled (i.e. the first one in the file is the one that is used and FM writes its own as the first). You can use the Microtype TimeSavers (see: FrameMaker-to-Acrobat TimeSavers and Improve PDFs: Set additional/custom XMP (Extensible Metadata Platform) fields automatically ) utility to make this easier from FM.