Based on this question ("What are the 'parts' of a SID procedure?"), I would like to ask for clarification concerning the role of the Common Route in a SID.
As far as I have understood, the most important part of the SID (the "runway transition" as it is called in the ARINC424 specification manual) is visualized with bold lines on the related charts. During this phase, the aircraft performs an obligatory climb with a minimum climb gradient of 200 ft/NM, unless otherwise specified. The remaning routes (common route and enroute transitions) are visualized with thinner lines and their respective Minimum Enroute Altitudes are also depicted.
What I find confusing is that the Common Route is sometimes visualized with bold lines and without Minimum Enroute Altitudes and other times with thinner lines and with their respective MEAs.
Is there a reason for having the Common Route sometimes as part of the Runway Transition (without MEAs) and other times as part of the Enroute Transitions (with MEAs)?
Τhe same distinction also applies to STARs, where common route can either be part of enroute or runway transitions. An example can be found here and here (OHSEA TWO Arrival, KSNA). Notice that common route is visualized with bold lines, same as runway transitions. However, most of the arrival procedures, as far as I can tell, use it as part of the enroute phase (containing MEA, drawn with thinner lines etc.).