Here is another table breakdown, that maybe better, then last one:
Main In-Portal tables
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1. PortalUser (list of users/admins, registered in system)
2. PortalGroup (list of groups, that can be assigned to registered users)
3. UserGroup (user-group associations)
4. PersistantSessionData (list of session-independent user settings; fix grammatical error in table name)
5. UserSession (list of sessions, produced by logged-in site visitors)
6. SessionData (data, associated with created user sessions)
7. Language (list of languages, defined in system)
8. LocalesList (list of supported locales, used for switching website language to user's browser language only)
9. CountryStates (list of countries with their states, used on user registration form)
10. Phrase (list of language labels of every language, defined in system)
11. Events (list of e-mail templates, that can be sent from the system)
12. Modules (list of modules, installed in system)
13. ConfigurationValues (list of configuration settings)
14. SearchConfig (list of search-related settings)
15. Category (list of pages, defined in system)
16. PageContent (content, that was entered in the pages, using Content Blocks)
17. CachedUrls (list of parsed & cached page mod-rewrite urls)
18. Semaphores (table, used in engine, that protects data from concurrent edit/save attempts by different users)
19. Theme (list of front-end themes)
20. ThemeFiles (list of front-end theme templates, used for mod-rewrite page url parsing)
21. Skins (list admin console skins)
22. PermCache (used only for advanced category-based permissions scheme, that can be disabled by default in future)
23. PermissionConfig (used only for advanced category-based permissions scheme, that can be disabled by default in future)
24. Permissions (list admin section permissions)
25. Agents (list of cronjobs)
In-Portal tables, that we can get rid of by peforming refactoring
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Cache (no need, when memcache is used)
IdGenerator (table, that keeps track of ResourceIds generated, get rid of it in future)
PhraseCache (merge with CachedUrls somehow)
PopupSizes (store in PortalUser.PopupSizes field/PersistentSessionData table as serialized array, we can have key, like md5(template_path) and popup size)
Counters (we only have 3 counters and can easily transfer them to memcache)
StatItem (delete)
In-Portal tables that can be moved into separate modules
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BanRules
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SessionLogs
ChangeLogs
CurlLog
SlowSqlCapture
StatisticsCapture
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CustomField
CategoryCustomData
PortalUserCustomData
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EmailQueue
EmailLog
MailingLists
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CategoryItems
Favorites
Images
ImportCache
ImportScripts
ItemFiles
ItemFilters
ItemRating
ItemReview
ItemTypes
SearchLog
RelatedSearches
CountCache
SpamReports
SpamControl
Relationship
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Drafts
FormFields
Forms
FormSubmissions
SubmissionLog
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SiteDomains
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Thesaurus
SpellingDictionary
StopWords