A key difference between the elaboration likelihood model $($ELM$)$ and the heuristic systematic model $($HSM$)$ is that:

The ELM focuses on responses to all types of messages, whereas the HSM focuses specifically on predicting responses to persuasive messages.

The HSM predicts that individuals can only process a message using either the central or peripheral route, whereas the ELM says it is possible to use both routes simultaneously.

The ELM predicts that individuals can only process a message using either the central or peripheral route, whereas the HSM says it is possible to use both routes simultaneously.

The HSM focuses on responses to all types of messages, whereas the ELM focuses specifically on predicting responses to persuasive messages.