Specify that the build-system table is optional

The 'requires' key in the build-system table is still required if the table is specified, though.

pep-0518.txt

@@ -21,11 +21,11 @@ Abstract
 ========
 
 This PEP specifies how Python software packages should specify what
-dependencies they have in order to execute their chosen build system.
-As part of this specification, a new configuration file is introduced
-for software packages to use to specify their build dependencies (with
-the expectation that the same configuration file will be used for
-future configuration details).
+build dependencies they have in order to execute their chosen build
+system. As part of this specification, a new configuration file is
+introduced for software packages to use to specify their build
+dependencies (with the expectation that the same configuration file
+will be used for future configuration details).
 
 
 Rationale
@@ -87,7 +87,7 @@ of issues, such as:
   may support different package formats or determine the latest
   version differently.
 
-This has cumulated in a situation where use of ``setup_requires``
+This has culminated in a situation where use of ``setup_requires``
 is rare, where projects tend to either simply copy and paste snippets
 between ``setup.py`` files or they eschew it all together in favor
 of simply documenting elsewhere what they expect the user to have
@@ -119,11 +119,11 @@ To provide more context and motivation for this PEP, think of the
 2. Installation of the build system.
 3. Execute the build system.
 
-This PEP covers step #2. It is fully expected that a future PEP will
-cover step #3, including how to have the build system dynamically
-specify more dependencies that the build system requires to perform
-its job. The purpose of this PEP though, is to specify the minimal set
-of requirements for the build system to simply begin execution.
+This PEP covers step #2. PEP 517 covers step #3, including how to have
+the build system dynamically specify more dependencies that the build
+system requires to perform its job. The purpose of this PEP though, is
+to specify the minimal set of requirements for the build system to
+simply begin execution.
 
 
 Specification
@@ -143,8 +143,11 @@ already in use by the Rust community as part of their
 Cargo package manager [#cargo]_ and in private email stated they have
 been quite happy with their choice of TOML. A more thorough
 discussion as to why various alternatives were not chosen can be read
-in the `Other file formats`_ section.
+in the `Other file formats`_ section. The authors do realize, though,
+that choice of configuration file format is ultimately subjective and
+a choice had to be made and the authors prefer TOML for this situation.
 
+Below we list the tables that tools are expected to recognize/respect.
 Tables not specified in this PEP are reserved for future use by other
 PEPs.
 
@@ -152,10 +155,10 @@ build-system table
 ------------------
 
 The ``[build-system]`` table is used to store build-related data.
-Initially only one key of the table will be valid and mandatory:
-``requires``. This key must have a value of a list of strings
-representing PEP 508 dependencies required to execute the build
-system (currently that means what dependencies are required to
+Initially only one key of the table will be valid and is mandatory
+for the table: ``requires``. This key must have a value of a list
+of strings representing PEP 508 dependencies required to execute the
+build system (currently that means what dependencies are required to
 execute a ``setup.py`` file).
 
 For the vast majority of Python projects that rely upon setuptools,
@@ -170,6 +173,23 @@ community at the moment, build tools are expected to use the example
 configuration file above as their default semantics when a
 ``pyproject.toml`` file is not present.
 
+Tools should not require the existence of the ``[build-system]``
+table. A ``pyproject.toml`` file may be used to store configuration
+details other than build-related data and thus lack a
+``[build-system]`` table legitimately. It is up to the tool to decide
+how to handle this case. Potential options are:
+
+1. Act as if the ``pyproject.toml`` file does not exist.
+2. Assume the table exists with the default values specified above.
+
+While subtle, there is a difference in these two options depending on
+whether the existence of a ``pyproject.toml`` file has special
+significance to the tool (e.g. pip currently triggers build isolation
+for a project when a ``pyproject.toml`` file is found). If help is
+desired in choosing an option for a tool then this PEP suggests the
+second option to help people move over to ``pyproject.toml`` files.
+
+
 tool table
 ----------
 
@@ -536,7 +556,7 @@ References
 
 .. [#jsonschema] JSON Schema
    (http://json-schema.org/)
-   
+
 .. [#file_formats] Nathaniel J. Smith's file format review
    (https://gist.github.com/njsmith/78f68204c5d969f8c8bc645ef77d4a8f)