A boarder then[ or now ] was someone who was charged for their board /food & lodging space .In the
past it could have been in a Boarding House run as a
business like all the guest houses in Douglas or living with
a family , as one of them, hopefully with their own bed but
probably sharing a room in small cottages & houses. Adults
would often be single people with no other home while children
might be left with these carers while their parent/s worked
somewhere children were not welcome. No social workers or
checkups then & it was an alternative to an orphanage.
In my own family I have a number of child boarders,one----
parent returned to take her to Canada eventually when a log
cabin was built/another father paid for motherless daughter to
live nearby--a miner & he too died ofTB & my g gran kept her
without any further payment----no checks at all--vicar/ schoolmaster would know she was safe & cared for.No legal
adoption in those times.