Server side boundary method
The server side boundary method makes sure that the server won't return too many results and is generally usefull at high zoom levels. It basically requests only the points in your currently visible area. If your at zoom level 1, where the map covers the entire globe we'll obviously again have too many returned points. That is why the returned points are limited to 20 ( you can actually change this limit to 10, 20, 50 or 100 in the map settings)
Because the server side method returns only a specific amound of markers (up to 100) if you have more markers in a specific category on the right in the menu one will have the option to navigate to the next page. This will create a new query that will return the next results for this category.
Due to the limited number of locations returned per query to some users it may seem that the locations are popping out of nowhere when they move around. That is why if you have more locations than the selected marker list length (in the map options) you'll need to combine this method with the custom tiles one