What is A1 - E?
These are your tileset sections. It means, whenever you create a map, each letter represents the things you can put through in your map and what layer they are. Trust me, there are still layers in RPG Maker MV. By layers, it means it's the priority where each of the tile appears inside your map. What does A1 and A5 mean specifically?
A1 is the animated tiles. These are the tiles that animates whenever you see them on the map. These usually consists of water, lava, falls,etc. These are the tiles that simulates the water, lava and falls as if they are alive. Of course, you need those in your games, don't you think? or else, if the water is static and stuff, it won't look very much appealing.
A2 consists of the Ground tiles. These would be the cemented ground, grassy ground, plain, desert, snow, etc.
A3 are the buildings, these would consist the roof of the buildings.
A4 are the buildings, these would consist of the walls of the buildings.
A5 would be the additional things in the buildings, such as the black and blank tiles, space tiles, ladder, stairs, carpeted ladder, etc.
B would be the decor, usually ranging from what is inside your map. For example, if you're mapping a room, then it should be the room decors and insides. Like the flower, the stairs, some cabinets, etc.
C onwards would be the things that are left to decor inside the room. Things that you think are not included in B.
Just remember, when mapping, B comes after C, C after D and D after E. So if you placed a C tile and you're trying to put the B tile, the C tile you placed just got erased. The only way to make that in reverse is make the tile an event.