The modified pages section

Database dumps reflect a snapshot of a database at a given moment in time. The archive database that represents a database dump is read-only. No user transactions are allowed. Although the archive database is read-only, some modifications are allowed. For example:

These modified and newly-allocated database pages cannot be stored within the database dump and its stripes, therefore an archive database requires some traditional database storage. This disk space is referred to as the modified pages section, and you can allocate it using the create archive database and alter database commands.