Pidgin Privacy options

[chris]

Hello,

I'm trying to understand the meaning of the Privacy options dialog 
(Pidgin 2.5.0 from getdeb.net).

I assume each option is probably has slightly different meanings for 
each protocol.

1. Allow all users to contact me
2. Allow only the users on my buddy list
3. Allow only the users below
4. Block all users
5. Block only the users below.

I use the following protocols: AIM, ICQ, IRC, MSN, XMPP

For each protocol, the following is selected by default (I have not 
modified them):

AIM: #4
ICQ: #4
IRC: #1
MSN: #3
XMPP: #1

So my questions are:

1) What determined which option would be the default for each protocol?
2) Why were they not all the same by default?
3) How did the list for MSN get populated, why did it not instead 
default to #2?  Is there a difference?
4) What do these options actually do?  Do they only prevent people from 
sending me a message, or does it block them from seeing my online status 
as well?  Does it block people from sending me authorization requests?

I've looked for a descriptive explanation of the Privacy options as they 
relate to each protocol and in general, but I was not able to find such 
a description.

I look forward to understanding the answer to at least some of these 
questions.

Regards,
chris