For all you guys ‘n dolls here an overview of the built-in apex variables and how/when to use ’em:
- APP_ALIAS
 - APP_ID
 - APP_IMAGES
 - APP_PAGE_ID
 - APP_SESSION
 - APP_UNIQUE_PAGE_ID
 - APP_USER
 - AUTHENTICATED_URL_PREFIX
 - BROWSER_LANGUAGE
 - CURRENT_PARENT_TAB_TEXT
 - DEBUG
 - HOME_LINK
 - LOGIN_URL
 - IMAGE_PREFIX
 - Application Express SCHEMA OWNER
 - PRINTER_FRIENDLY
 - LOGOUT_URL
 - PROXY_SERVER
 - PUBLIC_URL_PREFIX
 - REQUEST
 - SQLERRM
 - SYSDATE_YYYYMMDD
 - WORKSPACE_IMAGES
 
taken from this source
Variables can be referred to in three ways:
- Bind variable ( :APP_ID )
Used inside APEX in SQL and PL/SQL - V and NV functions ( NV(‘APP_ID’) )
Used outside APEX ( in PL/SQL packages for example ) - Substitution strings ( &APP_ID. )
Used inside APEX in items for example