Why is there a hole in the AC reservoir compartment?

The hole or slit is an air release port that opens when the com­part­ment is filled.

When the com­part­ment fills, the slit opens, and excess air vents out.  It pre­vents over­in­fla­tion or rup­ture of the com­part­ment.

LiveO2 AC Reservoir Vent Port.

LiveO2 AC Reservoir Vent Port