2. Properties Tab

First of all, give the new code injection a name which represents your needs.

With the Dashboard URL you could add a url to an external dashboard of the system you probably want to integrate. Keep in mind that this URL is not used anywhere else then the Tag Manager App, so it's probably not worth it to search for a appropriate url.

With the checkbox Activate on author you can tell the script that it should be triggered also in preview or editor mode. In most cases this will only add additional statistics and tracks your daily business. But it becomes very handy in testing sessions.

Tag location is a very necessary property. In the most external tools you will get a hint where you need to put your snippet into, this property is the way to choose it.