Salt n' Sauce

There's a strange phenomenon in Edinburgh and the surrounding area (possibly reaching as far as Fife) in that chip shops offer the option of "Salt n' Sauce" with your chips instead of Salt and Vinegar (although that IS also an option). The sauce is not your regular brown sauce you might have at home but a sort of half way between "regular" brown sauce and vinegar, I'm not sure exactly what it is, it may well be just "regular" brown sauce with added vinegar.

Anyway it tastes lovely on chips, I don't understand why it's not served elsewhere in the country.