When is semicolon is correct replacement of comma?

Hi. I think I sometimes see sentences in which a semicolon seems to be used in place of a comma for a reason I can imagine but can't know definitely. Can you help?

The document describes the people who attended the seminar yesterday; whether they are from out the city which the seminar was held; and the general life preferences such as what they would buy if they had an extra income available.

Note: the sentence was written (made up) to ask a grammar question and not to say anything

particular about a certain document.