user1878: I'm trying to write a BIP11-compliant C# client that creates a 2 of 2 or 2 of 3 transaction.
I'm also trying to understand if a 3 of 3 transaction is supported as well.
This example says they are creating a PS2H address, but I'm unsure of exactly what script is being signed, and the meaning of the parameters "2" in addmultisigaddress, and what happens when addmultisigaddress is called.
I have researched the BIP, the RPC documentation, and the forums.  I'm getting confused where if and when the P2SH is needed.

user18196: I think the reason you might be getting confused is that there are really two types of multisig, raw multisig and P2SH (pay-to-script-hash) multisig. P2SH multisig is basically just hashing the actual scriptPubKey desired and putting the scriptPubKey in the scriptSig. This stack exchange questions explains them in more detail: What are the limits of m and n in m-of-n multisig addresses?. 

I'm also trying to understand if a 3 of 3 transaction is supported as well.

Yes, you can do m-of-3 multisig transactions with both raw multisig and P2SH multisig.

the meaning of the parameters "2" in addmultisigaddress

The number provided is the m in the m-of-n. So if you want a 1-of-2, you do:
addmultisigaddress 1 '["1xy..", "1Jm..."]' 

The BIP11 (accepted) proposal you mentioned just made raw multisig a standard type of transaction that is recognized and forwarded on by standard clients. BIP16 (https://github.com/bitcoin/bips/blob/master/bip-0016.mediawiki) proposes the P2SH addresses, which allow for much shorter scriptPubKeys and support more types of multisig (e.g. 5-of-9, which you can't easily do with raw multisig because m-of-n multisigs with n > 3 are considered non-standard, as shown in the link in the first paragraph). 

1878: <p>I'm trying to write a BIP11-compliant C# client that creates a 2 of 2 or 2 of 3 transaction.</p>

<p>I'm also trying to understand if a 3 of 3 transaction is supported as well.</p>

<p><a href="https://people.xiph.org/~greg/escrowexample.txt" rel="nofollow">This example says they are creating a PS2H address</a>, but I'm unsure of exactly what script is being signed, and the meaning of the parameters "2" in addmultisigaddress, and what happens when addmultisigaddress is called.</p>

<p>I have researched the BIP, the <a href="https://en.bitcoin.it/wiki/Original_Bitcoin_client/API_calls_list" rel="nofollow">RPC documentation</a>, and the forums.  I'm getting confused where if and when the P2SH is needed.</p>
 18196: <p>I think the reason you might be getting confused is that there are really two types of multisig, raw multisig and P2SH (pay-to-script-hash) multisig. P2SH multisig is basically just hashing the actual scriptPubKey desired and putting the scriptPubKey in the scriptSig. This stack exchange questions explains them in more detail: <a href="https://bitcoin.stackexchange.com/questions/23893/what-are-the-limits-of-m-and-n-in-m-of-n-multisig-addresses/28092#28092">What are the limits of m and n in m-of-n multisig addresses?</a>. </p>

<blockquote>
  <p>I'm also trying to understand if a 3 of 3 transaction is supported as well.</p>
</blockquote>

<p>Yes, you can do m-of-3 multisig transactions with both raw multisig and P2SH multisig.</p>

<blockquote>
  <p>the meaning of the parameters "2" in addmultisigaddress</p>
</blockquote>

<p>The number provided is the m in the m-of-n. So if you want a 1-of-2, you do:</p>

<pre><code>addmultisigaddress 1 '["1xy..", "1Jm..."]' 
</code></pre>

<p>The BIP11 (accepted) proposal you mentioned just made raw multisig a standard type of transaction that is recognized and forwarded on by standard clients. BIP16 (<a href="https://github.com/bitcoin/bips/blob/master/bip-0016.mediawiki" rel="nofollow noreferrer">https://github.com/bitcoin/bips/blob/master/bip-0016.mediawiki</a>) proposes the P2SH addresses, which allow for much shorter <code>scriptPubKey</code>s and support more types of multisig (e.g. 5-of-9, which you can't easily do with raw multisig because m-of-n multisigs with n > 3 are considered non-standard, as shown in the link in the first paragraph). </p>
