Per the FAA’s “Chart Users Guide” , the bold route line is defined as the “Arrival Route” while the thinner route line is defined as the “Transition Route”. There is no reference that I can find which supports the concept that the heavy weight line is the section in which to expect vectors (and this is supported by the TANDY THREE ARRIVAL which has an extensive bold route line with no supporting textual note about expecting vectors).
In the Instrument Procedures handbook, some distinction is made between the “Arrival Route” and the “Transition Route”.
It then it goes on to say:The STAR officially begins at the common NAVAID, intersection, or fix where all the various transitions to the arrival come together. A STAR transition is a published segment used to connect one or more en route airways, jet routes, or RNAV routes to the basic STAR procedure. It is one of several routes that bring traffic from different directions into one STAR. This way, arrivals from several directions can be accommodated on the same chart, and traffic flow is routed appropriately within the congested airspace.
There are many cases where this naming convention is not consistently applied so it is not a surprise that this STAR is not named for HOMELAND. However, the KAYOH FOUR arrival clearly depicts the HDF-KAYOH segment as a “Transition Route”, not an “Arrival Route”. This is despite the fact that HDF is the common NAVAID where all the transitions come together. Presumably there is a reason that this segment is NOT considered to be part of the “Official STAR”.STARs usually are named according to the point at which the procedure begins. In the U.S., typically there are en route transitions before the STAR itself. So the STAR name is usually the same as the last fix on the en route transitions where they come together to begin the basic STAR procedure.
One thought that came to mind is that prior versions of the KAYOH arrival may have included additional transition routes which merged at KAYOH. I could not locate any older versions of the KAYOH STAR however a google search of “KAYOH THREE” resulted in this report from April of 1994 in the ASRS. In this report, an A320 flew the KAYOH THREE arrival and encountered multiple TCAS alerts from jump planes in the vicinity of HDF. Note that on the sectional, there is a Caution note referring to “Intensive Parachute activity” just west of HDF.
This highlights a point that a CFI which I respect a great deal taught me......when planning an IFR flight, always include a check of the your route on the VFR sectional in your flight planning.
I am now suspicious that there may be a legal nuance which allows the FAA to limit their liability for potential conflicts with jump activity by claiming that this leg is not officially part of the STAR. It seems odd for the FAA to distinguish between a “Transition Route” and an “Arrival Route”, without offering a more definitive definition of these terms (at least none that I can find). Perhaps there are different standards which a route segment must meet in order to be considered part of the “Arrival Route"?
EDIT: of course I overlooked the obvious answer which is that the bold line (the "Arrival Route") represents the route if no transition is called for. We focused so much on flying the transition that I totally overlooked the fact that this STAR is valid as HEC.KAYOH4, PSP.KAYOH4 or simply as KAYOH4. So the simple answer is that bold line is the route if no transition is specified in the clearance.