TYPO3 v8 has reached its end-of-life March 31st, 2020 and is not maintained by the community anymore.
You can order Extended Long Term Support (ELTS) here: TYPO3 ELTS.
Choosing an extension key¶
The "extension key" is a string uniquely identifying the extension. The folder where the extension resides is named by this string. The string can contain characters a-z0-9 and underscore. No uppercase characters should be used (keeps folder-,file- and table/field-names in lowercase). Furthermore the name must not start with an "tx" or "u" (this is prefixes used for modules) and because backend modules related to the extension should be named by the extension name without underscores, the extension name must still be unique even if underscores are removed (underscores are allowed to make the extension key easily readable).
The naming conventions of extension keys are automatically validated by the registration at the repository, so you have nothing to worry about here.
There are two ways to name an extension:
Project specific extensions (not generally usable or shareable): Select any name you like and prepend it "user_" (which is the only allowed use of a key starting with "u"). This prefix denotes that this extension is a local one which does not come from the central TYPO3 Extension Repository or is ever intended to be shared. Probably this is an "adhoc" extension you have made for some special occasion.
General extensions: Register an extension name online at the TYPO3 Extension Repository. Your extension name will automatically be validated and you are sure to have a unique name returned which nobody else in the world uses. This makes it very easy to share your extension later on with every one else, because it ensures that no conflicts with other extension will happen. But by default a new extension you make is defined "private" which means nobody else but you have access to it until you permit it to be public. It's free of charge to register an extension name. By definition all code in the TYPO3 Extension Repository is covered by the GPL license because it interfaces with TYPO3. You should really consider making general extensions!
It is far easier to settle for the right extension key from the beginning. Changing it later involves a cascade of name changes to tables, modules, configuration files etc. Think carefully.
About GPL and extensions¶
Remember that TYPO3 is GPL software and at the same moment you extend TYPO3 your extensions are legally covered by GPL. This does not force you to share your extension, but it should inspire you to do so and legally you cannot prevent anyone who gets hold of your extension code from using it and further develop it.The TYPO3 Extension API is designed to make sharing of your work easy as well as using others work easy. Remember TYPO3 is Open Source Software and we rely on each other in the community to develop it further.
It's also your responsibility to make sure that all content of your extensions is legally covered by GPL. The webmaster of TYPO3.org reserves the right to kick out any extension without notice that is reported to contain non-GPL material.
You are responsible for security issues in your extensions. People may report security issues either directly to you or to the TYPO3 Security Team. Whatever the case you should get in touch with the Security Team which will validate the security fixes. They will also include information about your (fixed) extension in their next Security bulletin. If you don't respond to requests from the Security Team, your extension will be forcibly removed from the TYPO3 Extension Repository.
More details on the security team's policy on handling security issues can be found at https://typo3.org/community/teams/security/extension-security-policy/.
Registering an extension key¶
Before starting a new extension you should register an extension key on typo3.org (unless you plan to make an implementation-specific extension – of course – which it does not make sense to share).
Go to typo3.org, log in with your (pre-created) username / password and go to Extensions > Extension Keys and click on the "Register keys" tab. On that page you can enter the key name you want to register.