Clarify "allow_system_table_mods"

I need clarification on allow_system_table_mods parameter
Per the documentation:
*Allows modification of the structure of system tables.* This is used by
initdb. This parameter can only be set at server start.

However, attempting to modify pg_class to add another column fails with
"STATUS_ACCESS_VIOLATION" as below.
So either only certain system catalogs may be changed, or only certain
types of structure changes are allowed.
eg: add indexes, triggers

Windows 10
PostgreSQL 9.2.15, compiled by Visual C++ build 1600, 32-bit

Current user is postgres
allow_system_table_mods = on

ALTER TABLE pg_catalog.pg_class ADD COLUMN relcreated timestamp DEFAULT
current_timestamp;

Fails with:

2016-04-25 12:08:00 EDT STATEMENT: ALTER TABLE pg_catalog.pgclass ADD
COLUMN relcreated timestamp DEFAULT current_timestamp;
2016-04-25 12:08:14 EDT LOG: server process (PID 7760) was terminated by
exception 0xC0000005
2016-04-25 12:08:14 EDT DETAIL: Failed process was running: ALTER TABLE
pg_catalog.pg_class ADD COLUMN relcreated timestamp DEFAULT
current_timestamp;
2016-04-25 12:08:14 EDT HINT: See C include file "ntstatus.h" for a
description of the hexadecimal value.
2016-04-25 12:08:14 EDT LOG: terminating any other active server processes

From ntstatus.h
*#define STATUS_ACCESS_VIOLATION ((NTSTATUS)0xC0000005L) //
winnt*

--
*Melvin Davidson*
I reserve the right to fantasize. Whether or not you
wish to share my fantasy is entirely up to you.