Develop
Allow unsigned extensions, and set where new extensions go and what their ids start with.
For people who write their own extensions. Make an extension, switchboard ext new and Extract to Extension all use these settings.
What's here #
Develop mode has Allow unsigned extensions. Extensions you write yourself aren't signed by the gallery. With this on they can be installed, and they run in the same sandbox as any other, with only the permissions they list. Turn it off to refuse everything unsigned.
New extensions has the Id prefix and the Folder. A new extension called "Word Count" gets an id built from the prefix and the name, shown live under the field. Ids are reverse-DNS and unique, so use a domain or a name that is yours. Choose… picks a different folder and Show in Finder opens it.
Tricks #
Changing the prefix affects extensions you make from then on. Existing ones keep their ids.
If the engine is off when you flip Develop mode, the page says it was saved and applies at the next start.