About
Internal (or Administration) is an administration application aimed to manage workspace instance. (Ie one Apex installation can have several workspace)
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Management
Link
In the login page
Api
A workspace instance can be changed through PL/SQL code with the APEX_INSTANCE_ADMIN package. It can be executed by the SYS, SYSTEM, and APEX_040200 database users and any database user granted the role APEX_ADMINISTRATOR_ROLE.