I’ve been helping people apply for spousal visas on this forum for over 11 years, and of the thousands of approvals I’ve seen, no one has ever needed the SU07 form.
It is a form specifically for special circumstances visas, namely adult dependant relatives and elderly dependant relatives, where the applicant is physically incapable of taking care of themselves (they need their U.K. sponsor to care for them, to feed, bathe and clothe them), they cannot work, they have no money of their own, have no means of care in their home country, and therefore are 100% financially dependant on their U.K. sponsor.
Which is where the SU07 comes in, where the sponsor takes full financial responsibility for the applicant for the next 5 years. Very, very few people can qualify for this type of visa.
From the UKVI website:
3. ECB12.3 Sponsor undertakings on maintenance and accommodation
As with undertakings from an applicant (see above), the ECO should only require an undertaking from a sponsor in exceptional circumstances or for certain categories eg parents, grandparents and other dependent relatives under Rule 317 of the Immigration Ruleshttps://www.gov.uk/government/publications/undertakings-ecb12/ecb12-undertakings#ecb123-sponsor-undertakings-on-maintenance-and-accommodationSent from my iPhone using Tapatalk
