Sector code explanation

[Nevermore-demise]

Hello.
I'd be very thankful if someone could explain me how does the server know to which sector of a map a point belongs to, and also how are sectors stored in memory(as an unidimensional array or as a two dimensional one)
I've been looking through the code, but I can't figure it out. I get lost at this part.

		private Sector InternalGetSector( int x, int y )
		{
			if ( x >= 0 && x < m_SectorsWidth && y >= 0 && y < m_SectorsHeight )
			{
				Sector[] xSectors = m_Sectors[x];

				if ( xSectors == null )
					m_Sectors[x] = xSectors = new Sector[m_SectorsHeight];

				Sector sec = xSectors[y];

				if ( sec == null )
					xSectors[y] = sec = new Sector( x, y, this );

				return sec;
			}
			else
			{
				return m_InvalidSector;
			}
		}

Thanks