The define tag in the Content section assigns the template name.

This tag is optional, and when omitted, the template name is derived from the Name field.

When both are specified, it is a best practice to ensure that they are the same.

ref https://grafana.com/docs/grafana/latest/alerting/unified-alerting/message-templating/

default template https://github.com/grafana/grafana/blob/main/pkg/services/ngalert/notifier/channels/default_template.go