Hi Mark,
Please note that I haven't used the OpenACS, but I think the answers to your questions are common to both the OpenACS and ACS Classic so I will give it a stab.
"For example what is [ad_parmeter SpecialIndexPage content] or [ad_header[ad_system_name] ? Is there a document that describes what these things are and what they are used for?"
ad_parameter and ad_header are both procedures defined in the file /web/[your_service_name]/packages/acs-core/defs-procs.tcl. You can find a searchable directory of every procedure defined in the ACS installed on your site by going to the page http://[www.your-web-address.com]/doc/procs. For example, the definitions for the procs for the arsdigita.com site are available at http://www.arsdigita.com/doc/procs.
"[ad_parameter SpecialIndexPage content]" will return the value of the SpecialIndexPage configuration parameter in the content subsections of your your-service-name.ini file.
[ad_header[ad_system_name]] creates a header for the page with system name as the page title. See the docs for details.
You can find documentation on ADP's in Chapter 2 of the AOLserver Tcl Developer's Guide, available at http://www.aolserver.com/doc/3.0/tcldev/tcldev.htm.
A sequence is simply a sequence of numbers. Often you will want to have a unique number to use as a key for a row in a table. Many RDBMS's provide a built-in sequence generator. For example, to create a sequence in Oracle:
SQL> create sequence foo_sequence;
Sequence created.
SQL> select foo_sequence.nextval from dual;
NEXTVAL
----------
1
For more information about sequences, see the Transactions chapter in SQL for Web Nerds at
http://www.arsdigita.com/books/sql/transactions.html
Chris
