What is a "System of Engagement" layer actually 

Every enterprise has invested heavily in its Systems of Record. The financials live in the ERP. Employees live in the HR system. Assets live in the CMDB. These systems are accurate, governed and where the truth lives.

The System of Engagement layer is where humans actually interact with all those systems of record. The layer where a manager approves a purchase. Where a new employee gets onboarded. Where an exception gets flagged, routed, and resolved. Where work, real and judgment-dependent, actually happens.