You asked the question in terms of ARINC 424 which imposes the constraints of the database structure and use. So I'll try to explain SIDs within that constraint and in the broader sense. The reason for that is that ARINC 424 exists to support the use of an FMS (or an RNAV navigator). But some SIDs don't require an FMS to fly them. The ones that start with "SID" or Vector". FMS SIDs require and FMS (but not necessarily a modern RNAV capable FMS). RNAV SIDs require an RNAV system with the database because they use RNAV waypoints that only exist within the database. A modern RNAV FMS can use any of the SIDs.
For the more generic description of SIDs, I refer to the Instrument Procedures Handbook (FAA-H-8083-16B).
As described there and in 757toga's answer, a SID has one or more runway transitions (the start point) and a route (a sequence of fixes) to the end of the basic DP. For some SIDs, this is the enroute transition. For others, there are additional transition routes that fan out from the end of the basic DP to multiple enroute transition fixes.
Enroute transition fixes are fixes that exist in the SID and in the enroute system structure (e.g., on the Low Altitude IFR charts).
So back to what you are seeing in ARINC 424. Each of the SID routes is coded in the database starting with the Runway Transition, followed by the waypoints that define the Route and ending at the Enroute Transition.
When entering the flight plan into the FMS, you select the depart runway. You can then insert a SID. If there is more than one enroute transition, the FMS will require you to select the enroute transition. You can then add the enroute portion of your plan.
Since SIDs can cover many environments
What the FMS does with what you entered is to start the plan from a fix on the selected runway with a 'takeoff' leg, typically a FA leg (fix to an altitude) to 400 feet AGL. The FMS then 'strings' in the SID with a leg to the runway transition followed by legs to each of the fixes in the SID route until it reaches the enroute transition. While it does this it adds the altitude constraints to each of the fixes along the route. The enroute portion of the flight plan is then added.
You'll note that "Vector" SIDs only list a Runway Transition and an Enroute Transition as the route will be provided by ATC as vectors. The FMS loads these two waypoints separated by a DISCO (disconnect). It will sequence the runway transition after takeoff and then do nothing while the pilot follows the ATC vectors. When told by ATC, the pilot can clear the DISCO by entering a Direct-To leg to the Enroute Transition fix.