Many ports and terminals impose a deadweight restriction on visiting ships, and they ensure that this is not breached by requiring the marking of a set of load lines with the Summer mark indicating the draught for a deadweight below the restriction limit.
Where the restriction is 25, 000 dwt, for example, the Summer load line will usually be marked at the draught for 24, 999 dwt; for a restriction of 50, 000 dwt, the Summer load line will be at 49, 999 dwt. Some ships have several sets of marks, with a Load Line Certificate issued by the classification society for each set. None of these additional sets of load line marks may be higher, however, than the set required for the minimum freeboard assigned under the International Load Line Rules.
The Rules only allow one set to be in use at any one time, so that a ship may arrive at a port using one set, but sail using another. Any marks not actually in use should be painted out to hide them.