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Template variables come from migrations.vars in your config. If you set cluster_name on the environment, it is also available as {{cluster_name}}.
If you have used Atlas, this will feel familiar. The idea is the same as Atlas templated migrations, but clisma uses Handlebars instead of Go templates. See the Handlebars docs for syntax examples.

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Environment-specific templating

You can define template variables per environment in migrations.vars to make the same migration work differently across environments. A common case is: local runs standalone tables, while production runs a replicated cluster. In that case you can set is_replicated, create_table_options per environment and let the same SQL adapt at runtime. 
env "local" {
  url = "http://default:password@localhost:8123/mydb"

  migrations {
    dir = "migrations"
  }
}

env "production" {
  url = env("CLICKHOUSE_PROD_URL")
  cluster_name = "prod-cluster"

  migrations {
    dir = "migrations"
    vars = {
      is_replicated = true
      create_table_options = "ON CLUSTER prod-cluster"
    }
  }
}

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Example migration

CREATE TABLE IF NOT EXISTS events {{create_table_options}} (
  id UUID,
  created_at DateTime
)
{{#if is_replicated}}
ENGINE = ReplicatedMergeTree('/clickhouse/tables/{cluster}/events', '{replica}')
{{else}}
ENGINE = MergeTree()
{{/if}}
ORDER BY id;
Note: {cluster} and {replica} are ClickHouse macros, not clisma template variables. See the ClickHouse macros docs. You can replace them with your own values if you want to avoid macros.