Evidence to support an application where a person has been absent from the UK for more than 2 years may include:
evidence of previously holding Indefinite Leave to Remain (ILR) in the UK
details of any family in the UK and correspondence with them (to establish strong ties to the UK)
evidence of property in the UK and/or any business interests (to establish strong ties to the UK)
letters of enrolment/attendance at an education establishment if they have been studying outside the UK for long periods
letter of employment where this has been the reason for their absence from the UK
a letter from a medical professional if their reason for their absence relates to caring for another person or for their own medical reasons
This list is not exhaustive. Other evidence can be taken into account, and depending on your circumstances, you may need to provide additional documents.