Here we're only interested in

Synonymize descriptions ? - select synonyms, if select then what % of words to replace with synonyms (% words to Synonymize ).

The file of synonyms syn.txt can be seen in Settings - Config Files, but it's recommended to edit it via FTP, because it's more than 2.2M in size and editing it via web is inconvenient. Physically the file is located in scj/includes/conf