diff --git a/format/spec.md b/format/spec.md
index 5a90f6fd978d..07adcb63d0f0 100644
--- a/format/spec.md
+++ b/format/spec.md
@@ -150,6 +150,10 @@ Readers should be more permissive because v1 metadata files are allowed in v2 ta
 
 Readers may be more strict for metadata JSON files because the JSON files are not reused and will always match the table version. Required v2 fields that were not present in v1 or optional in v1 may be handled as required fields. For example, a v2 table that is missing `last-sequence-number` can throw an exception.
 
+##### Writing data files
+
+All columns must be written to data files even if they introduce redundancy with metadata stored in manifest files (e.g. columns with identity partition transforms). Writing all columns provides a backup in case of corruption or bugs in the metadata layer.
+
 ### Schemas and Data Types
 
 A table's **schema** is a list of named columns. All data types are either primitives or nested types, which are maps, lists, or structs. A table schema is also a struct type.
@@ -241,7 +245,14 @@ Struct evolution requires the following rules for default values:
 
 #### Column Projection
 
-Columns in Iceberg data files are selected by field id. The table schema's column names and order may change after a data file is written, and projection must be done using field ids. If a field id is missing from a data file, its value for each row should be `null`.
+Columns in Iceberg data files are selected by field id. The table schema's column names and order may change after a data file is written, and projection must be done using field ids.
+
+Values for field ids which are not present in a data file must be resolved according the following rules:
+
+* Return the value from partition metadata if an [Identity Transform](#partition-transforms) exists for the field and the partition value is present in the `partition` struct on `data_file` object in the manifest. This allows for metadata only migrations of Hive tables.
+* Use `schema.name-mapping.default` metadata to map field id to columns without field id as described below and use the column if it is present.
+* Return the default value if it has a defined `initial-default` (See [Default values](#default-values) section for more details). 
+* Return `null` in all other cases.
 
 For example, a file may be written with schema `1: a int, 2: b string, 3: c double` and read using projection schema `3: measurement, 2: name, 4: a`. This must select file columns `c` (renamed to `measurement`), `b` (now called `name`), and a column of `null` values called `a`; in that order.