Norwegian (Bokmal) to English Tech/Engineering Ships, Sailing, Maritime

In price lists for some Norwegian harbours the term "intervall BT" (BT = bruttotonnasje) is found. Det brukes ifm. kaivederlag og ansløpsavgift.



Example: http://www.altahavn.no/priser.39533.no.html

Here it states: "Det er forskjellige satser etter fartøysgruppe en tilhører. Minimumsbeløp for kaivederlag er kr. 200.-" Then it gives a table where the heading "Intervall BT" appears.

But what is the equivalent term in English? "tonnage class/group" or something like that? It's not "interval" that's for sure.