When you add a target, Probely will only scan pages under that target, i.e., it will not scan any pages from a different hostname.
This is what the setting "Add extra hosts" is for. If you have an api residing on a different hostname, you can add it there. This way, our scanner will scan the API as well.
Technically speaking, what this option does, is:
Any XMLHttpRequest performed to a host added under this setting, is followed and scanned.