Hi Adrian,
It sounds like you got it yes.
The dialer interface is the logical interface where we configure all protocol stuff. Things like PPP, CHAP/PAP authentication, etc.
A dialer interface can only belong to one dialer pool.
In the dialer pool, we can have one or more physical interfaces…physical interfaces can belong to one or more dialer pools. That’s totally cool if you want to use it.
When you use something like PPPoE, then it’s normally the CPE that “initiates” the connection. When the router has a packet with the dialer interface as the next hop, then this will trigger the dialer to become active. A default route is a good example yes.
It’s called a dialer because originally this was all used for analog modems / ISDN lines where we used dialing 
Hope this helps!
Rene