I think it's a more tricky situation. It still can be seen I believe as subletting but the cut seems less clear.
Subletting is a form of lease. Lease typically involves contractual agreement with the payments. On the other hand, if one is letting to somebody a part of the property
for free it can be seen as making a donation. How would it be different from receiving a payment and giving it back? And it may also involve taxation.
If we omit the payment thing, subletting would be just letting somebody to possess a part of the property.
Obviously there is also time and relationship factor. If I let my friend to stay with me for a couple of days, I don't think anybody would consider it subletting. But if I require that friend to pay me for the stay, well, I think it could be seen as subletting or other form of short term rental.
Just speculating, I am not a lawyer.