Code P22DH(M):

P = Type of tank:

Tank, battery-vehicle or MEGC for liquefied gases or dissolved gases.
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22 = Calculation pressure:

Minimum calculation pressure: 22 bar
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D = Openings (see 6.8.2.2 and 6.8.3.2):

Tank with top filling or discharge openings with 3 closures; or battery-vehicle or MEGC with no openings below the surface of the liquid.
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H = Safety valves/devices:

Hermetically closed tank, battery-vehicle or MEGC (see 1.2.1).
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(M) = The substance can also be carried in battery-vehicles or MEGCs.