Relationships can have attributes which modify the relationship. The attributes which are available depend on the relationship type. How attributes should be used also depends on the relationship type, so please see the documentation for the relationship you want to use for more information.
Most attributes are simply boolean, they are either selected or not selected. The instrument, vocal, license and medium attributes however are lists which can be extended. New entries can be proposed for these attributes, see the proposals process for more details.
The full list of attributes is available on the server.
All relationships have two date fields, a begin date and an end date, to store the period of time during which the relationship applied. The date can be the year, the year and the month or the full date. It is optional, so it can also be left blank. As with other attributes, see the documentation for the relationship types you are using.