I have received some clarifying information from developers. It should be useful for you:
First, I would to note that actually Registration module could be used to organize password protected area of web site. Yes, it's true that there is no way to do that in UI, but one could achieve that by editing web.config file of the published site. Of course it's bad idea to edit site's root web.config file because it will be rewritten by the publication procedure, but since asp.net looks for web.config file in each subfolder too, following trick can be used:
1. Create subfolder to organize password protected area
2. In that subfolder place web.config file with following content
<configuration>
<system.web>
<authorization>
<deny users="?" />
<allow users="*" />
</authorization>
</system.web>
</configuration>
That is access to pages in this subfolder will be restricted to authenticated users only.
Regarding to the reuse of Registration module data by other applications.
Sitebuilder 4.5 modules don't know about APS standard and thus do not provide any integration points to use by means of the APS.
But good news is that Registration module use standard asp.net 2.0 mechanism to deal with users registration and authorization tasks. As one may note looking at web.config Registration module implements asp.net membership provider, thus any .Net code run in the same web site may use standard asp.net features to obtain information about registered users. Actually all Sitebuilder modules reuse registration module data in this way. Moreover, they may extend user's profile data with additional fields as necessary. This is configured by App_Data\profile.config file.
<?xml version="1.0"?>
<profile enabled="true" defaultProvider="StorageProfileProvider">
<providers>
<clear />
<add name="StorageProfileProvider" type="SWsoft.SiteBuilder.Modules.Storage.StorageProfileProvider, SiteBuilder.Modules.Storage" />
</providers>
<properties>
<add name="FirstName" />
<add name="LastName" />
<add name="Company" />
<add name="Phone" />
<add name="Address1" />
<add name="Address2" />
<add name="Country" />
<add name="City" />
<add name="State" />
<add name="Zip" />
<group name="Forum" />
<group name="Guestbook">
<add name="HomepageUrl" type="String" />
</group>
</properties>
</profile>
You may find that Guestbook module extends profile metadata by adding HomepageUrl field. Actual data is stored in Sitebuilder.mdb file. There are four tables in database related to this functionality: MembershipApplication, MembeshipInfo, MembershipUser, ProfileData.
But, it's not recommended to access this tables directly and the best way is to use Membership providers infrastructure.