Application Express includes a number of built-in substitution strings. These provide information that can be used in HTML regions or blocks of PLSQL code.

The strings are:

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
SCHEMA_OWNER
PRINTER_FRIENDLY
LOGOUT_URL
PROXY_SERVER
PUBLIC_URL_PREFIX
REQUEST
SQLERRM
SYSDATE_YYYYMMDD
WORKSPACE_IMAGES

If you wish to display the contents of these strings on a test page add the following to an HTML region :

<b>APP_ALIAS</b>: &APP_ALIAS.<br />

<b>APP_ID</b>:  &APP_ID.<br />

<b>APP_IMAGES</b>:  &APP_IMAGES.<br />

<b>APP_PAGE_ID</b>:  &APP_PAGE_ID.<br />

<b>APP_SESSION</b>:  &APP_SESSION.<br />

<b>APP_UNIQUE_PAGE_ID</b>:  &APP_UNIQUE_PAGE_ID.<br />

<b>APP_USER</b>:  &APP_USER.<br />

<b>AUTHENTICATED_URL_PREFIX</b>:  &AUTHENTICATED_URL_PREFIX.<br />

<b>BROWSER_LANGUAGE</b>:  &BROWSER_LANGUAGE.<br />

<b>CURRENT_PARENT_TAB_TEXT</b>:  &CURRENT_PARENT_TAB_TEXT.<br />

<b>DEBUG</b>:  &DEBUG.<br />

<b>HOME_LINK</b>:  &HOME_LINK.<br />

<b>LOGIN_URL</b>:  &LOGIN_URL.<br />

<b>IMAGE_PREFIX</b>:  &IMAGE_PREFIX.<br />

<b>SCHEMA_OWNER</b>:  &SCHEMA_OWNER.<br />

<b>PRINTER_FRIENDLY</b>:  &PRINTER_FRIENDLY.<br />

<b>LOGOUT_URL</b>:  &LOGOUT_URL.<br />

<b>PROXY_SERVER</b>:  &PROXY_SERVER.<br />

<b>PUBLIC_URL_PREFIX</b>: &PUBLIC_URL_PREFIX.  <br />

<b>REQUEST</b>:  &REQUEST.<br />

<b>SQLERRM</b>:  &SQLERRM.<br />

<b>SYSDATE_YYYYMMDD</b>:  &SYSDATE_YYYYMMDD.<br />

<b>WORKSPACE_IMAGES</b>:  &WORKSPACE_IMAGES.<br />

You'll notice the syntax for including in an HTML block is

&<substitution_string>.

You can also include them in PLSQL page processes :

v('substitution_string')

or by prefixing the substitution string with a colon

:APP_ID

For additional information see the official APEX documentation.