Add docs about flow_published_at metadata field (#1483)

site/docs/concepts/collections.md

@@ -38,7 +38,43 @@ An example document for a collection with two fields, `name` and `count` is show
 }
 ```
 
-The `_meta` object is present in all Flow documents, and contains metadata added by Flow. Minimally, every document `_meta` always has a `uuid`, which is a globally unique id for each document. Some capture connectors may add additional `_meta` properties to tie each document to a specific record within the source system. Documents that were captured from cloud storage connectors, for example, will contain `/_meta/file` and `/_meta/offset` properties that tell you where the document came from within your cloud storage bucket.
+### System Fields and Metadata
+
+The `_meta` object is present in all Flow documents, and contains metadata added by Flow.
+Minimally, every document `_meta` always has a `uuid`, which is a globally unique id for each document.
+Some capture connectors may add additional `_meta` properties to tie each document to a specific record
+within the source system. Documents that were captured from cloud storage connectors, for example,
+will contain `/_meta/file` and `/_meta/offset` properties that tell you where the document came from
+within your cloud storage bucket.
+
+#### _meta/uuid
+
+The `_meta/uuid` field is a system-generated globally unique identifier for each document within Estuary Flow.
+
+#### flow_published_at
+
+The `flow_published_at` field is a system-generated timestamp within Estuary Flow,
+derived from the runtime environment. It captures the exact moment a document is published to a collection,
+offering a reliable proxy for when the document was last modified or inserted.
+
+- Source: The `flow_published_at` field is generated by the runtime environment of Estuary Flow.
+- Definition: This field represents the timestamp when a document is captured and subsequently published
+to a collection. Essentially, it is a projection of the `_meta/uuid` field, where the UUID contains an
+encoded timestamp component.
+- Availability: The `flow_published_at` field is available in every collection, as it is a derived
+projection from the `_meta/uuid` field.
+
+For a given document identified by a unique key, the `flow_published_at` field can be used as a proxy for
+the last time the document was modified. This is particularly useful when performing incremental updates
+or transformations, such as in a data warehouse environment.
+
+When dealing with materializations that are _not_ delta updates:
+
+- A document in Estuary Flow is any JSON object emitted by a capture connector.
+The `flow_published_at` field provides the timestamp for when this JSON object was captured and inserted
+into the collection.
+- If the collection is reduced with a strategy like `lastWriteWins` or `merge` on the
+capture side, `flow_published_at` becomes the timestamp for the last event that updated the document.
 
 ## Viewing collection documents