so you have p_data[33] starting from 0 to 32 (the last is EOS, if you will use p_data[33] = 15 you will get an index out of bounds in server console).
players id are starting from 1 to 32, that means p_data[1] is the first player that join the server and so on.
your variable is defined p_data[33][P_LAST] where P_LAST = 24 (last value that can t be used), the numbers from 0 that 23 are powers. for texample you want to print in console the current xp of the player that joined on the server and has id 13.
not using the defined macros, you should use the variable like p_data[13][6] and you will get his XP.
in this case, the author of the plugin used the macros to not memorize numbers for every power so he will use in code p_data[13][P_XP]