Siphon is a long bent pipe which is used to transfer liquid from a reservoir at a higher elevation to lower elevation when the two reservoirs are separated by a hill or high level ground.
The point C which is at the highest of the siphon is called summit as shown in fig. As the point C is above the free surface of the water in the tank A, the pressure at C will be less than atmospheric pressure.
Theoretically, the pressure at C may be dropped to -10.3m of water but in actual practice this pressure is only -7.6m of water or 10.3-7.6=2.7m of water absolute.
If the pressure at C becomes less than 2.7m of water absolute then dissolved and other gases would come out from water and collect at the summit.