A resource manager is simply a program with some well-defined characteristics.
This program is called different things on different operating systems—some call them “device drivers,” “I/O managers,” “filesystems,” “drivers,” “devices,” and so on. In all cases, however, the goal of this program (which we'll just call a resource manager) is to present an abstract view of some service.
Also, since QNX Neutrino is a POSIX-conforming operating system, it turns out that the abstraction is based on the POSIX specification.