Consider a replica set with three nodes: A, B, C. 'A' is PRIMARY, 'B' and 'C' are SECONDARY.
If there is a network partition between 'A' and 'C', then:
- A is still PRIMARY because it can see a majority of the set
- C will start an election trying to become PRIMARY
- It can see a majority of the set
- It cannot see a primary
If, for whatever reason, 'B' allows 'C' to become PRIMARY, then the set will have two PRIMARY nodes until one of them is stepped down.