Originally secondary legislation on Scottish matters was made in the UK Statutory Instrument series and the fact that the legislation applied to Scotland was denoted by a subseries of the UK series. When devolution occurred most of these functions were transferred to Scottish institutions and the SSI series was created. However there are still some matters which impact on Scotland that are reserved matters within the meaning of the Scotland Act; those powers are exercised by the UK ministers through the UK SI series.
Before devolution there were typically around 200 SIs a year in the Scotland subseries in the main UK series and now there are between 10 and 20 a year. There are typically around 500 or 600 SSIs made in a year.