Hermetically sealed means vapour-tight closure. For certain substances IMDG Code packing provisions require package to be hermetically sealed.
For example Packing provision PP31 under instruction P400 states “For UN Nos 2870, packagings shall be hermetically sealed”
Unless otherwise specified for a particular substance in IMDG Code, packages containing dangerous goods which evolve flammable gases or vapour, may become explosive if allowed to dry, evolve toxic gases or vapour, evolve corrosive gases or vapour or may react dangerously with the atmosphere should be hermetically sealed.
IMDG Code Compliance Centre 