Holds the input URL and parameters for an individual track
When the input is a playlist, a p# object is created to hold the URL and parameters for one track in the playlist. The # is the position of the track in the playlist (starting from 1).
The contents of this object depend on what the client specified when calling mmr_track_parameters() for this track. The attributes that this object may contain are the same as those that can be found in the input object.