They were usually members of a merchant guild.
Previous to the Reform Act of 1832 those who paid scot and bore lot were often entitled to the franchise. The expression used today originated from this time period. Members that did not pay their taxes "got off 'scot-free' ".
The phrase is preserved in the Disorderly Houses Act of 1751, which empowers inhabitants of a parish or place paying scot and bearing lot therein (i.e. ratepayers) to require the constable of the parish to prosecute disorderly houses.
See D. P. Fry, "On the Phrase Scot and Lot", in Trans. Philological Society (1867), pp. 167-197; C. Gross, Gild Merchant, i. c. iv.; Pollock and Maitland, Hist. Eng. Law, p. 647.