It’s purely stylistic, but here are the rules - https://en.wikipedia.org/wiki/Long_s#Rules

Some of the rules for the use of the long s from https://en.wikipedia.org/wiki/Long_s

Long s was always used (ſong, ſubſtitute), except:

• Upper-case letters are always the round S; there is no upper-case long s.
• A round s was always used at the end of a word ending with ⟨s⟩: his, complains, ſucceſs
  • However, long s was maintained in abbreviations such as ſ. for ſubſtantive(substantive), and Geneſ. for Geneſis(Genesis).
• Before an apostrophe (indicating an omitted letter), a round s was used: us’d and clos’d.
• Before or after an f, a round s was used: offset, ſatisfaction.

It looks almost like the old german “S”.