You can have multiple routing tables (similar to VRFs on networking gear) which are classified into routing domains and assign different rdomains to different applications. See rdomain(4): https://man.openbsd.org/rdomain.4
man rdomain to start, though unfortunately you kind of need to already understand rdomains to fully understand the docs. Basically it's a network partitioning/virtualization tool. Two computers can have two routing tables. rdomains lets one computer have two routing tables. Each process is in one rdomain or another which determines where its traffic goes and how it sees the network.
It sounds like someone wanted /net and private namespaces from Plan 9, but they were stuck with Posix, and this is the catenary track they built to make the square wheels run smoothly.