FOC stands for: Free of charge.
It is often used in the travel industry to describe (additional) services for which the client does not have to pay for.
An upgrade into a higher room category is often FOC (for promotional purposes, as added value, in line with complaint management, in line with loyalty programs, in case of overbookings in lower room categories, etc.).
Other examples can be: Free Wifi in a Hotel, Free FAM Trip for a journalist /Press Tour