Code CxBN(M):

C = Type of tank:

Tank, battery-wagon or MEGC for compressed gases.
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x = Calculation pressure:

Value of the minimum relevant test pressure according to the table in 4.3.3.2.5.
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B = Openings (see 6.8.2.2 and 6.8.3.2):

Tank with bottom filling or discharge openings with 3 closures; or battery-wagon or MEGC with openings below the surface of the liquid or for compressed gases.
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N = Safety valves/devices:

Tank, battery-wagon or MEGC with safety valve according to 6.8.3.2.9 or 6.8.3.2.10 which is not hermetically closed.
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(M) = The substance can also be carried in battery-wagons or MEGCs.