[1][2] It has also been used colloquially to refer to more general differences in policy between Scotland and England due to Scottish devolution.
The term itself could relate to an Independent Scotland's physical border with the rest of the UK as well as differences in the currency and membership of international organisations such as NATO.
Part of the issue relates to the Schengen treaty, which allows free movement between European Union member states.
The United Kingdom exercised an opt out from this treaty in order to maintain complete control over its own border.
The House of Commons Foreign Affairs Committee report entitled "The foreign policy implications of and for a separate Scotland" contains the following quote as Written evidence from the Foreign and Commonwealth Office: "The impact of an independent Scotland joining Schengen, were it to wish or be obliged to do so, would be significant, creating a UK land border with the Schengen area for the first time (the Republic of Ireland is not a member of Schengen).