Where do the symbols in the CSA system notation come from?

Network symbols (firewall, router, switch, LAN, load balancer) are borrowed from network topology diagrams, process symbols (activity, start and end points, decision) from UML, and application symbols cover user interfaces, servers, databases, services, identity providers, cloud services and the internet; a few symbols, such as the WAF and VPN router, are specific to this notation.

* The full symbol set. Starred symbols exist only in this notation. *

The notation deliberately reuses what readers already know rather than inventing a new standard:

• Network symbols from network topology diagrams: firewall, router, switch, LAN, load balancer, plus the notation-specific web application firewall (WAF) and VPN router.
• Application symbols: user interface, server, database, service, identity provider (IdP), cloud service, internet, processing.
• Process symbols from UML: process, activity, start point, end point, decision, exit point. The exit point marks the hand-over to another process.

The set is meant to be extended as needed. The point is quick, shared understanding, not formal completeness.

Go deeper:

• Cisco — Network topology icons — the de facto icon set of network diagrams that the network symbols come from.
• Activity diagram — Wikipedia — the UML origin of the start, end and decision symbols.