Answer
It isn't a fixed global IFS setting — the start-of-week boundary used to resolve a relative week-based substitution variable is derived dynamically from the browser/client locale in use at the time, so the exact same variable resolves to a different date range depending on which locale the browser is set to (for example, a Sunday-start range for a US-locale browser versus a Monday-start range for many European locales), rather than being controlled by a single system-wide IFS configuration value.