"... software on operational systems.", as part of the description of control A.12.5.1 (Installation of software on operational systems) of ISO 27001 Annex A, refers to any software that is used in normal daily operations of an organization. This includes either software used on datacenters (e.g., database management systems, corporate backup software, etc.), as well as software used on employees workstations (e.g., e-mail clients, word processors, etc.).