diff --git a/documentation/Sphinx/developer_guide/index.rst b/documentation/Sphinx/developer_guide/index.rst
index 7e2f43d6..8ffde519 100644
--- a/documentation/Sphinx/developer_guide/index.rst
+++ b/documentation/Sphinx/developer_guide/index.rst
@@ -19,3 +19,4 @@ Vernier.
    dev_process
    testing
    review_process
+   post_processing
diff --git a/documentation/Sphinx/developer_guide/post_processing.rst b/documentation/Sphinx/developer_guide/post_processing.rst
new file mode 100644
index 00000000..b48e222b
--- /dev/null
+++ b/documentation/Sphinx/developer_guide/post_processing.rst
@@ -0,0 +1,34 @@
+.. -----------------------------------------------------------------------------
+    (c) Crown copyright 2026 Met Office. All rights reserved.
+    The file LICENCE, distributed with this code, contains details of the terms
+    under which the code may be used.
+   -----------------------------------------------------------------------------
+
+Post Processing Design
+======================
+
+The post processing is delivered in Python, independent of the code calipering functionality.
+
+The post processing is implemented as a library, which can beb used by Python programmes, and as a set of callable scripts.
+
+Callable Script Design Constraints
+----------------------------------
+
+Callable scripts enable the outputs from Verniered runs to be parsed without writing code.
+
+For maintainability and ease of use reasons, these are limited by design constraints:
+
+* Inputs only: locations (file path) 
+* Outputs only: text to stdout only 
+
+Python Module Design Constraints
+--------------------------------
+
+The importable Python modules enable programmatic interaction with data parsed from Vernier output files.
+
+For maintainability reasons, these are limited by design constraints:
+
+* Enable parsing of Vernier structured data files.
+* Enable organsiation and aggregation of Vernier data.
+* No plotting, ever (this is user responsibility).
+* Functionality shall be tested.
diff --git a/documentation/Sphinx/user_guide/output/interpreting-output.rst b/documentation/Sphinx/user_guide/output/interpreting-output.rst
index acba3bcf..5c092e5e 100644
--- a/documentation/Sphinx/user_guide/output/interpreting-output.rst
+++ b/documentation/Sphinx/user_guide/output/interpreting-output.rst
@@ -8,8 +8,8 @@ Interpreting Output
 -------------------
 
 By default Vernier will write information out into one file per MPI rank.
-Single file output is not currently supported but is achievable via
-post-processing techniques.
+Single file output is supported via the environment variable:
+``VERNIER_OUTPUT_MODE=single``
 
 Each file contains a table of the all the declared regions and their
 associated walltimes.
@@ -76,4 +76,4 @@ To reiterate, the two output format options are **threads** and **drhook**.
 * The self and total time per call (in ms) is also given.
 
 In both examples the ``@0`` appended onto the end of all region names indicates
-the OpenMP thread number.
\ No newline at end of file
+the OpenMP thread number.