IR35 and CIS solve different tax-compliance questions. One tests deemed employment through an intermediary, the other governs deductions from subcontractor payments in construction.
IR35 is the label commonly used for the off-payroll working rules. It matters where someone provides services through an intermediary and the arrangement may resemble employment.
CIS is a tax deduction scheme for construction work. Contractors may need to verify subcontractors and make deductions from payments before passing those amounts to HMRC.
Construction supply chains combine agencies, subcontractors, and project delivery pressures. That makes IR35 less about theory and more about workflow discipline.