There was some lengthy discussion about that question a year or two ago. The conclusion was that many of us had created our own practices for using the "technical layers".
"CrtYd" (courtyard) is intended to help the board designer arrange components on the board. The lines on this layer show the maximum physical extent of the actual component, plus a clearance allowance for automated component placement machinery, and possibly clearance around large components to mitigate their "thermal shadowing" effects in some reflow ovens. (Hey, even with manual placement you need a few dozen mils between parts to get your tweezer tips between them.) The intention is that the courtyard lines of adjacent components may touch, but not overlap.
"Fab" (Fabrication) is an ambiguous name - I probably would call it something like "Assy". In KLC, it's used to show actual component outlines, polarity marks, values, etc. It's intended to show the assembler which components are mounted at various locations on the board, and how they are oriented. It may also be used for assembly notes, to generate illustrations for, e.g., test procedure documents, troubleshooting guides, maintenance manuals, etc. Prior to the most recent KLC I had already started using ECO1 for this purpose.
"Dwgs.User" (User Drawings) is the layer I use for communication with the PWB etch-and-drill company. On it I place dimensions for the physical outline, plus any cutouts or special mechanical details. I also place all the details regarding construction of the bare board - thickness, layer stackup, copper weight, soldermask color, etc. In the "good old days" (formerly known as "these trying times", before they got a different marketing company) the board fabricators used a drawing like this as a basis for quoting a job. Receiving Inspection used this drawing to make sure we got what we ordered. Mechanical Design used this drawing to make paper cutouts for planning (or confirming) enclosure design, front panel layout, etc. Now we pass most of this information by clicky-clicky with a computer mouse . . . . and a week later try to remember whether we ordered 1 oz or 2 oz copper; whether it was the second prototype or the third that switched to red soldermask; whether the SMT version was built on a high-temperature substrate or not; etc.
Here's a sample "Dwg.User" from a very simple board I knocked out last week: