from the docs of APM: https://www.elastic.co/guide/en/apm/get-started/7.12/transactions.html
Transactions are a special kind of span that have additional attributes associated with them. They describe an event captured by an Elastic APM agent instrumenting a service. You can think of transactions as the highest level of work you’re measuring within a service. As an example, a transaction might be a: ...
How is a "service" defined?