diff --git a/docs/proposed/psa-driver-interface.md b/docs/proposed/psa-driver-interface.md
index 563bd97ca..de336147a 100644
--- a/docs/proposed/psa-driver-interface.md
+++ b/docs/proposed/psa-driver-interface.md
@@ -5,7 +5,7 @@ This document describes an interface for cryptoprocessor drivers in the PSA cryp
 
 This specification is work in progress and should be considered to be in a beta stage. There is ongoing work to implement this interface in Mbed TLS, which is the reference implementation of the PSA Cryptography API. At this stage, Arm does not expect major changes, but minor changes are expected based on experience from the first implementation and on external feedback.
 
-Time-stamp: "2020/08/06 18:52:00 GMT"
+Time-stamp: "2020/08/06 18:56:39 GMT"
 
 ## Introduction
 
@@ -46,7 +46,7 @@ The present specification was designed to fulfill the following high-level requi
 
 ### Deliverables for a driver
 
-To write a driver, you need to implement some functions with C linkage, and to declare these functions in a **driver description file**. The driver description file declares which functions the driver implements and what cryptographic mechanisms they support. Depending on the driver type, you may also need to define some C types and macros in a header file.
+To write a driver, you need to implement some functions with C linkage, and to declare these functions in a **driver description file**. The driver description file declares which functions the driver implements and what cryptographic mechanisms they support. If the driver description references custom types, macros or constants, you also need to provide C header files defining those elements.
 
 The concrete syntax for a driver description file is JSON. The structure of this JSON file is specified in the section [“Driver description syntax”](#driver-description-syntax).
 
@@ -70,7 +70,7 @@ A driver description is a JSON object containing the following properties:
 
 * `"prefix"` (mandatory, string). This must be a valid prefix for a C identifier. All the types and functions provided by the driver have a name that starts with this prefix unless overridden with a `"name"` element in the applicable capability as described below.
 * `"type"` (mandatory, string). One of `"transparent"` or `"opaque"`.
-* `"headers"` (optional, array of strings). A list of header files. These header files must define the types provided by the driver and may declare the functions provided by the driver. They may include other PSA headers and standard headers of the platform. Whether they may include other headers is implementation-specific. If omitted, the list of headers is empty.
+* `"headers"` (optional, array of strings). A list of header files. These header files must define the types, macros and constants referenced by the driver description. They may declare the entry point functions, but this is not required. They may include other PSA headers and standard headers of the platform. Whether they may include other headers is implementation-specific. If omitted, the list of headers is empty.
 * `"capabilities"` (mandatory, array of [capabilities](#driver-description-capability)).
 A list of **capabilities**. Each capability describes a family of functions that the driver implements for a certain class of cryptographic mechanisms.
 * `"key_context"` (not permitted for transparent drivers, mandatory for opaque drivers): information about the [representation of keys](#key-format-for-opaque-drivers).