Unbound threads are created PTHREAD_SCOPE_PROCESS. These threads are scheduled in user space to attach and detach from available LWPs in the LWP pool. LWPs are available to threads in this process only; that is threads are scheduled on these LWPs.
In most cases, threads should be PTHREAD_SCOPE_PROCESS. This allows the threads to float among the LWPs, and this improves threads performance (and is equivalent to creating a Solaris thread in the THR_UNBOUND state). The threads library decides, with regard to other threads, which threads get serviced by the kernel.