Hi Fedor,
Thanks for reaching out. You could expect to see the 'UNKNOWN' type when it is not one of these types: SEARCH, DISPLAY, SHOPPING, VIDEO, MULTI_CHANNEL, EXPRESS. The 'UNKNOWN' type is mostly for the purpose to extend the features of Google Ads API as some campaign types may be already available on the Google Ads UI but still not supported in the API. By using 'UNKNOWN' type it provides a way to differentiate those types from the supported types. Please let us know if you have any further concerns.
Thanks and regards,
Xiaoming, Google Ads API Team