Difference between “Set as custom master page ” and “set as default master page”

Now when I created a new master page for my site collection based on seatle.master, I have two options for the new .master page inside sharepoint desinger; “Set as custom master page” & “Set as default master page”. Not sure if my understanding for these options are correct:-

1. 
   

   Set as default, means any new page will be based on this master page. While if i have old pages they will still use the old master page.

   
   


2. 
   

   Set as custom. Means that all old pages will be based on this custom master page.

   
   

Can anyone advice ?

Thanks