Source: https://databento.com/docs/api-historical/basics/schemas-and-conventions



### Schemas and conventions

A schema is a data record format represented as a collection of different
data fields. Our datasets support multiple schemas, such as order book,
trades, bar aggregates, and so on. You can get a dictionary describing
the fields of each schema from our [List of market data schemas](https://databento.com/docs/schemas/whats-a-schema.md).

You can get a list of all supported schemas for any given dataset using
the Historical client's [list_schemas](https://databento.com/docs/api-historical/metadata/metadata-list-schemas.md) method. The same information can also be
found on the dataset details pages on the [user portal]($PLATFORM_HOSTNAME).

The following table provides details about the data types and conventions used
for various fields that you will commonly encounter in the data.

| Name | Field | Description |
| Dataset | `dataset` | A unique string name assigned to each dataset by Databento. Full list of datasets can be found from the [metadata](https://databento.com/docs/api-historical/metadata/metadata-list-datasets.md). |
| Publisher ID | `publisher_id` | A unique 16-bit unsigned integer assigned to each publisher by Databento. Full list of publisher IDs can be found from the [metadata](https://databento.com/docs/api-historical/metadata/metadata-list-publishers.md). |
| Instrument ID | `instrument_id` | A unique 32-bit unsigned integer assigned to each instrument by the venue. Information about instrument IDs for any given dataset can be found in the [symbology](https://databento.com/docs/api-historical/symbology.md). |
| Order ID | `order_id` | A unique 64-bit unsigned integer assigned to each order by the venue. |
| Timestamp (event) | `ts_event` | The matching-engine-received timestamp expressed as the number of nanoseconds since the [UNIX epoch](https://en.wikipedia.org/wiki/Unix_time). |
| Timestamp (receive) | `ts_recv` | The capture-server-received timestamp expressed as the number of nanoseconds since the [UNIX epoch](https://en.wikipedia.org/wiki/Unix_time). |
| Timestamp delta (in) | `ts_in_delta` | The matching-engine-sending timestamp expressed as the number of nanoseconds before `ts_recv`. See [timestamping](https://databento.com/docs/architecture/timestamping-guide.md) guide. |
| Timestamp out | `ts_out` | The Databento gateway-sending timestamp expressed as the number of nanoseconds since the [UNIX epoch](https://en.wikipedia.org/wiki/Unix_time). See [timestamping](https://databento.com/docs/architecture/timestamping-guide.md) guide. |
| Price | `price` | The price expressed as signed integer where every 1 unit corresponds to 1e-9, i.e. 1/1,000,000,000 or 0.000000001. |
| Book side | `side` | The side that initiates the event. Can be **A**sk for a sell order (or sell aggressor in a trade), **B**id for a buy order (or buy aggressor in a trade), or **N**one where no side is specified by the original source. |
| Size | `size` | The order quantity. |
| Flag | `flag` | A bit field indicating event end, message characteristics, and data quality. |
| Action | `action` | The event type or order book operation. Can be **A**dd, **C**ancel, **M**odify, clea**R** book, **T**rade, **F**ill, or **N**one. |
| Sequence number | `sequence` | The original message sequence number from the venue. |