A 'source' indicates the geographical origin of a reference. Sources may be towns and cities, regions, river basins, lakes, seas, plateaus, mountain ranges and so forth. A reference that sources from a wide area is presumed to originate from scattered mines, shops or fields, rather than from a tightly organized guild.
Sources are organized by markets into zones - preferably, there should be one market per zone. As an example, look at the small region of Sanseverino, in Italy, below:
The source table above shows the number of references originating within the Principality of Sanseverino in Southern Italy, a region of about 500 sq.miles located upon the Gulf of Salerno. There are two population centers, the largest by far being Salerno, a large city of 32,815 people. Eboli, in comparison, has only 906 - and notably does not appear in the above table.
This does not mean that Eboli produces nothing. Note that the table includes a line for 'Sanseverino' - this refers to the whole region. We may assume from the table that both Eboli and Sanseverino, as well as tiny rural communities surrounding them, produce cheese, olive oil, wine, paper and so on. Salerno itself also produces foodstuffs (that is, it processes the maize, oats, wheat, fruits and so on of the region), cloth, smelting (making metal from ore), olives and grapes. It also - within the city and the immediately surrounding fields - as many vegetables as the whole rest of the region, shown by the additional 1 reference.
The 'Sele Basin' refers to a river bottom in the region, where additional livestock are raised. And one additional center - not a town or city, San Valentino - makes pitch. Because it has no significant population, San Valentino would be a monastery, tar pit or similar concern.
For convenience, the source table has been color-coded for different kinds of product: green for made foods, blue for textiles, orange for timber and wood products, purple for chemicals, red for metals, yellow for staple products, teal for fruits & vegetables, brown for livestock and blue for fish. As Sanseverino is on sea, the fish would be salt-water fish; were the fish to originate in the Sele Basin, we could presume freshwater.