FBP imposes a strict accounting on IPs - all IPs received or created by a process must be disposed of explicitly, much as one disposes of a memo: the IP can be sent on, destroyed or filed. An IP can also be attached to another one, much as one would clip two memos together. This can ramify into a tree structure which is sent from process to process as a single unit (see TreeIP), so "ownership" can be broadened to be direct or indirect.
In the various implementations of FBP to date, which all used a single address space, ownership was transferred by shipping IP addresses or handles around - from a logical point of view it doesn't matter if one transfers the whole IP or just a handle - this is just a performance issue. Between address spaces or different hardware processors, it makes more sense to transfer the whole IP.