The entire historical and religious point / purpose behind the
Camino de Santiago from the very beginning, around 850 AD, is to make pilgrimage to the crypt under the main altar at the Cathedral in Santiago to venerate the remains / relics of the Apostle Saint James the Greater, also the Patron Saint of Spain. Yes, the
Camino de Santiago originated as a primarily Christian activity.
Remember that, pre-reformation in the early 1500's, there was only one Christian church. And that Church was and continues to be ruled by the Pope, in Rome. Everything else we know as Camino-related evolved from that Papal-pronounced premise.
Thus, all formal
Camino de Santiago routes terminate at Santiago. This is also why it is not called Camino de Finisterre or Camino de Muxia, etc. I am not trying to be sarcastic, just historically accurate.
I am ONLY stating this as a basis for stating that any other walk, with a destination other than Santiago is not, strictly speaking, a route of the
Camino de Santiago. Of course, a person who is walking anywhere for any other reason can do whatever they please. But, it is a hike, not a Camino.
However, and all this said, you can only qualify for a Compostela or Certificate of Distance if your pilgrimage ends at Santiago and you have walked the final 100 km on foot, or have ridden the last 200 km on bicycle.
Also, and in case you wondered, a Compostela is a logical prerequisite for obtaining the Certificate of Distance. The distance certificate attests to the pilgrim having travelled a specific distance (in km) having started at "A" and arrived at Santiago. As one is not a "pilgrim" until certified as such by receiving a Compostela, the mileage certificate is a secondary document. You must first have received a Compostela before being able to obtain the distance certificate. The Compostela remains free. There is a nominal "donation" for the distance certificate...three Euros if I recall correctly.
So, while it is possible to cobble segments of various routes together into a marvelous hike across Spain, it is NOT a Camino unless it terminates at Santiago. it would be more properly construed as a long-distance hike following parts of various Camino routes.
Finally, and this is the important bit, it IS POSSIBLE to combine parts of other Camino routes and arrive at Santiago by a non-traditional route, but still qualify for the Compostela by meeting the final 100 / 20o km (foot / bike) proviso.
For example, we regularly had folks arriving at the Pilgrim Office who combined portions of the Camino del Norte, the Primitivo, or the Invierno, feeding into the Frances, to arrive at Santiago. Others started at southern points and combined portions of other Caminos in a similar fashion. But, they all walked the final 100 km, or rode the final 2200 km on bicycle, to arrive at Santiago, satisfying the requirement for the Compostela. In addition, the new, "spiritual variant" on the Camino Portuguese is another recent innovation in this context.
Also, this year, we had a number of pilgrims, arriving at Santiago from Muxia, via Finisterre. They walked the Muxia / Finisterre / Santiago route in reverse. Alone, neither the Muxia - Santiago; nor, the Finisterre - Santiago route meet the minimum distance requirement. However, in combination, they are just over 100 km. I understand this was accepted as being valid for obtaining a Compostela.
I hope this helps the dialog.