PlayerSelectors implements all the default Minecraft selectors:

• @a - All players
  
• @p - Nearest player
  
• @e - Entities
  
• @r - Random player
  
• @s - Self/Sender
  
  but also features a custom made one:
  Since PocketMine implements a multi-world system, you can easily select players from your current world using the "@w" selector!
  
  Minecraft also supports specific search parameters such as the distance from the sender, the count of matched player/entities, ...
  All the standard Minecraft search params can be used with the syntax

@<selector name>[key=param,other=value]

EXCEPT the team & score params since the /scoreboard command isn't implemented into PocketMine yet
Though, one new param that comes with this plugin is the "lvl" param, which is for searching in a specific world.

Have fun!