The Application ID is the string "$(click_package)_$(application)_$(version)" (without the quotes and with the obvious substitutions made).

`click_package` is the dotted name of the click package; a package `com.ubuntu.foo_0.5_all.click` would have `click_package` of `com.ubuntu.foo`.

`application` is the hook name from your `manifest.json` file (obtainable for an installed package with `click info com.ubuntu.foo`). The `manifest.json` will contain a section like: `"hooks": { "myapphookname": { "desktop": "FooApp.desktop" } }`. The `application` here is `myapphookname`.

'''version''' is the installed version number.

This ID is used for identifying the application throughout the system:

 * Click
 * Mir
 * HUD
 * [[../AppArmorProfile|AppArmorProfile]]
 * Content Hub
 * Download manager
 * Infographic (libusermetrics)
 * etc

The following restrictions to the components apply, from various parts of the system:

||               || filesystem || store^1^       || apparmor^2^       || deb^3^^,^^4^          || intersection        ||
|| click_package || [^\0/]+    || [a-zA-Z0-9.-]+ || [a-zA-Z0-9+.:~-]+ || [a-z0-9][a-z0-9+.-]+  || [a-z0-9][a-z0-9.-]+ ||
|| application   || [^\0/]+    ||                || [a-zA-Z0-9+.:~-]+ || [a-zA-Z0-9+.-]+       || [a-zA-Z0-9+.:~-]+   ||
|| version       || [^\0/]+    || [0-9.]+        || [a-zA-Z0-9+.:~-]+ || [0-9][a-zA-Z0-9.+:~-]*|| [0-9][0-9.]*        ||
 0. The store supports debian versioning (ie, same as the 'deb' entry) but the !AppStore interface uses a reduced scheme to keep it simple for users
 0. [[https://wiki.ubuntu.com/SecurityTeam/Specifications/ApplicationConfinement/Manifest#Security_keys|security manifest definition]]
 0. [[http://bazaar.launchpad.net/~click-hackers/click/trunk/view/head:/doc/file-format.rst|click file-format definition]]
 0. [[http://bazaar.launchpad.net/~click-hackers/click/trunk/view/head:/doc/hooks.rst|click hooks definition]]

Additionally, version should be structured more or less as you would expect, and be strictly monotonically increasing (new versions should always be greater than old versions).

Alternate nomenclatures:
 * 'APP_ID' and '`${id}`' (from click documentation) are synomymous with '!ApplicationId'. The full regex for a valid APP_ID is:{{{
[a-z0-9][a-z0-9+.-]+_[a-zA-Z0-9+.-]+_[0-9][a-zA-Z0-9.+:~-]*
}}}
 * 'click package name' ("name" in the click manifest) and 'APP_PKGNAME' are synonymous with $(click_package)
 * 'application name' and 'appname' are synonymous with $(application)
 * 'APP_VERSION' is synonymous with $(version)

'''Note:''' as of 2013-09-18 only one $(application) is allowed per click package. Future versions will support specifying multiple $(application)s per click package (this is already supported in the click manifest and click-apparmor, where the keys to the click manifest's hooks dictionary define the values for each $(application)).