Standard maps, also known as normal maps, are categorized as P0, run in normal rooms and have a standard duration of 2 minutes.
At the end of a P0 map, players can vote if they liked the map or not. If the map has over 100 votes and the vote is below 50%, the map is moved to the deleted (P44) category. The chat command /info [code] displays the total number of votes and approval percentage a map has received.
When the map editor was first introduced (V0.63), normal rooms only played two map categories: the original maps made by Tigrounette and user-made maps (P0). As people started to make maps, the category of user-made maps became massive. And of course, some maps were better than the others. At this point P1 was introduced, to keep the better maps in the rotation for a longer period and also to give them more playtime.
P0 shares its place in the normal map rotation with P1, which means these maps are pooled together and the game randomly selects one of these maps from this pool to be played. To make sure P1 gets a decent chance of being played in a room (and to reduce server load), this pool doesn't contain every single P0 map available. According to Sydoline, only the oldest 40,000 P0s are included. Which means that at any given time, the pool of P0s the game gets to pick from is the oldest 40,000 P0s at the last restart and the new P0s exported since the restart. At the next restart, the oldest 40,000 are chosen again and the newly exported maps are no longer included in the pool. Also note that /info on any map not in the given 40,000 returns a "Map not found".