Migration from 8.2 to 8.4 or major version migration and how postgres db is handled?

http://www.postgresql.org/docs/8.4/static/migration.html 

and still it is not clear for me. The question is that

http://www.postgresql.org/docs/8.4/static/migration.html  states first do a

pg_dumpall  (which contains postgres db) Just in case I have made

pg_dumpall --clean --inserts (--insert is not required though) on Pgsql 8.2

Then on target machine which contains 8.4 I have issued

psql -f backup postgres 

command.

My question here is that with import command my backup file contains postgres db which may NOT be compatible with 8.4 since this 

dump is for 8.2

So how this incompatibility is resolved? AFAIK major versions are not compatible and requires dump/restore but how postgres table (aside from other db that we have created) is handled?

Regards.