I'm going through the Trailhead training (here), and see the following two snippets which seem to disagree:
"It’s possible to change a lookup relationship to a master-detail relationship, but not the other way around. Hence, if you’re in doubt which one to use, it’s best to start with a lookup relationship. If you later decide you need the features of a master-detail relationship, you can change it.
And later on the same page I find:
"If you delete the primary master-detail relationship or convert it to a lookup relationship, the secondary master object becomes primary."
These seem at odds to me, so either (1) I'm misunderstanding what it's saying or (2) there is a discrepancy in the documentation. Which is it? If I'm just misunderstanding, can someone explain what it really means?