RussellSpitzer commented on code in PR #16425:
URL: https://github.com/apache/iceberg/pull/16425#discussion_r4446408551


##########
format/spec.md:
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@@ -1801,6 +1804,40 @@ If a delete column in an equality delete file is later 
dropped from the table, i
 
 Manifests hold the same statistics for delete files and data files. For delete 
files, the metrics describe the values that were deleted.
 
+### Column Files
+
+Starting in v4, a data file may store fields in column files listed in 
[`column_files`](#column-file-fields). A column file can use any supported 
Iceberg file format and must have rows that align with the data file it 
modifies.
+
+A column file is responsible for all the values of the fields listed in its 
`field_ids`. Each ID must identify a field in the table schema, or a row 
lineage field required by [Writing Column Files](#writing-column-files). A 
schema field must be top-level or nested in structs, and must not be nested 
within a list or map.

Review Comment:
   @anuragmantri 
   
   Notes from today's sync. In order to support updates to column-files structs 
themselves, we want to make sure we can replace just about anything.
   
   The proposed change was to say that any schema field can be replaced, but 
only the entire field. An entire list must be replaced, not just parts of it - 
the same with Maps. 



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