To me, the question was essentially about the meaning of implicit, and explicit, and not in the specific context. The fact it didn't have any context, except the link to the external site, which could be lost anytime the link would return a 404, or 403 error page, lead me to think the question was not constructive, and too generic.
I noticed the question when I was handling a flag raised for that question from a user who thought it was not constructive.
I have re-opened the question. I am more than happy to give anybody the opportunity of giving a detailed answer.
The question is given for a specific context I think is related to Drupal: relationships between Drupal entities. It can be explained with the database theory, but that doesn't make the question less related to Drupal.