This seems like a mixture of Watcher context variables, and Kibana Alerting context variables. Watcher and Kibana Alerting not related in anyway, but have some similarities, such as calling these variables (generally) context variables. For watcher, they generally start with {{ctx.***}} and for Kibana Alerting, they start with {{context.***}}.
For Kibana alerting, the context variables depend on the alerting rule type itself. For instance, here's a list of variables made available by the index threshold alerting rule type: Index threshold | Kibana Guide [7.13] | Elastic
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