Either "spanning" or "split", to specify how this extension will behave if allowed to run in incognito mode.
The default for extensions is "spanning", which means that the extension will run in a single shared process. Any events or messages from an incognito tab will be sent to the shared process, with an incognito flag indicating where it came from. Because incognito tabs cannot use this shared process, an extension using the "spanning" incognito mode will not be able to load pages from its extension package into the main frame of an incognito tab.
The default for installable web apps is "split", which means that all app pages in an incognito window will run in their own incognito process. If the app or extension contains a background page, that will also run in the incognito process. This incognito process runs along side the regular process, but has a separate memory-only cookie store. Each process sees events and messages only from its own context (for example, the incognito process will see only incognito tab updates). The processes are unable to communicate with each other.
As a rule of thumb, if your extension or app needs to load a tab in an incognito browser, use split incognito behavior. If your extension or app needs to be logged into a remote server or persist settings locally, use spanning incognito behavior.