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

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