The difference is from functionnality point of view, but it is very small.
In S&T, POI are generally referred as the built in Database of Point of Interest (1.6 millions i guess?). This database include restaurant, museum, and many different things. You can search in S&T (Find) in this database. You may see the internal POI database as an index (alphabetical sorted list of things) of some name you can find on a physical paper map (but the index includes many things museum, city hall, etc but not street names). You can find around a position on the map to display all found POI in the surrounding and display them on the map.
Pushpins in S&T can be seen as a physical pushpin you can use to place it on a physical paper map. It is like a real pushpins you can place on the map (to mark an interesting spot).
So when you are importing file from the outside, you may call them as you like, but once imported in S&T, they are becoming a pushpins set, similar as the one you are using on a paper map, but instead of placing just one at a time, you are setting a bunch of pins in one single shot.
But these pushpins once imported are not part of the POI database of S&T, they are 2 differents things as explained above.
I hope this help