api.md update and rename (#676)

* api.md: update for ItemClassification

* world api.md: rename from api.md

docs/adding games.md

@@ -8,7 +8,7 @@ There are two key steps to incorporating a game into Archipelago:
 
 Refer to the following documents as well:
 - [network protocol.md](https://github.com/ArchipelagoMW/Archipelago/blob/main/docs/network%20protocol.md) for network communication between client and server.
-- [api.md](https://github.com/ArchipelagoMW/Archipelago/blob/main/docs/api.md) for documentation on server side code and creating a world package.
+- [world api.md](https://github.com/ArchipelagoMW/Archipelago/blob/main/docs/world%20api.md) for documentation on server side code and creating a world package.
 
 
 # Game Modification  
@@ -337,6 +337,7 @@ fields in the class being extended.
 This is also a good place to put game-specific quirky behavior that needs to be managed, as it tends to make things a bit
 cluttered if you put these things elsewhere.  
   
-The various methods and attributes are documented in `/worlds/AutoWorld.py[World]`, 
+The various methods and attributes are documented in `/worlds/AutoWorld.py[World]` and
+[world api.md](https://github.com/ArchipelagoMW/Archipelago/blob/main/docs/world%20api.md),
 though it is also recommended to look at existing implementations to see how all this works first-hand. 
 Once you get all that, all that remains to do is test the game and publish your work.

docs/world api.md

@@ -114,14 +114,21 @@ Special locations with ID `None` can hold events.
 Items are all things that can "drop" for your game. This may be RPG items like
 weapons, could as well be technologies you normally research in a research tree.
 
-Each item has a `name`, an `id` (can be known as "code"), and an `advancement`
-flag. An advancement item is an item which a player may require to advance in
-their world. Advancement items will be assigned to locations with higher
+Each item has a `name`, an `id` (can be known as "code"), and a classification.
+The most important classification is `progression` (formerly advancement).
+Progression items are items which a player may require to progress in
+their world. Progression items will be assigned to locations with higher
 priority and moved around to meet defined rules and accomplish progression
 balancing.
 
 Special items with ID `None` can mark events (read below).
 
+Other classifications include
+* filler: a regular item or trash item
+* useful: generally quite useful, but not required for anything logical
+* trap: negative impact on the player
+* skip_balancing: add to progression to skip balancing; e.g. currency or tokens
+
 ### Events
 
 Events will mark some progress. You define an event location, an
@@ -346,7 +353,7 @@ from .Options import mygame_options  # the options we defined earlier
 from .Items import mygame_items  # data used below to add items to the World
 from .Locations import mygame_locations  # same as above
 from ..AutoWorld import World
-from BaseClasses import Region, Location, Entrance, Item, RegionType
+from BaseClasses import Region, Location, Entrance, Item, RegionType, ItemClassification
 from Utils import get_options, output_path
 
 class MyGameItem(Item):  # or from Items import MyGameItem
@@ -453,7 +460,9 @@ from .Items import is_progression  # this is just a dummy
 def create_item(self, item: str):
     # This is called when AP wants to create an item by name (for plando) or
     # when you call it from your own code.
-    return MyGameItem(item, is_progression(item), self.item_name_to_id[item],
+    classification = ItemClassification.progression if is_progression(item) else \
+                     ItemClassification.filler
+    return MyGameItem(item, classification, self.item_name_to_id[item],
                       self.player)
 
 def create_event(self, event: str):