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).