Understanding how to choose between fields and tags in InfluxDB

What are some good rules and examples of how to choose between storing data in fields vs. tags when designing [InfluxDB schemas](https://docs.influxdata.com/influxdb/v1.2/concepts/schema_and_data_layout/) ? What I've [found so far](https://docs.influxdata.com/influxdb/v1.7/concepts/schema_and_data_layout/#encouraged-schema-design) is: > a measurement that changes over time should be a field, and metadata about the measurement should be in tags > > tags and fields are effectively columns in the table. tags are indexed, fields are not > > the values that are highly variant and not usually part of a WHERE clause are put into fields > > Store data in fields if you plan to use them with an InfluxQL function > > Tags containing highly variable information like UUIDs, hashes, and random strings will lead to a large number of series in the database, known colloquially as high series cardinality. High series cardinality is a primary driver of high memory usage for many database workloads. But let's say you store filled orders in an e-commerce application: order id, sale price, currency. * Should the order id be a tag, or a field? * Should the currency be a tag, or a field?