The tag relation with id relation defines a related location. The value is stored in the column REL. The value should be the location id of the related location.
if, for example, the related location of location A changes over time than this should be configured in the following way. In this example we will assume that location A has a related location B
until 1-1-2014. After this date the related location should be location C. First a row should exists in the dbf file of this locationset with the value A in column ID and the column which defines the relation (REL) should have the id
B of location B. The END column should indicate that this is valid until 1-1-2014. A second row with the same value A for the ID column should exists. The START column should indicate that this row is valid from 1-1-2014.
The column REL should have the value C to indicate that the related location is C.