“Periodic Reminder” — System, rules and principles
“How RPG Rules Work” — fiction, players and rules
“Rules vs Vigorous Creative Agreement” — Rules exist to force outcomes that no-one playing would have chosen
His use of “system” in those follows the Lumpley Principle / Baker-Care Principle — “System (including but not limited to ‘the rules’) is defined as the means by which the group agrees to imagined events during play.”1
NB I generally don’t use that definition — it conflicts with the much more general use that I’m familiar with professionally, and it is unhelpful as a jargon term because it’s too easily confused with “rules” or “ruleset”. So I use a much more generic definition as sketched in Some meanings of some terms.