See this post for details:
Can't see all the monitors
http://devcentral.f5.com/Default.aspx?tabid=53&aft=1172093&afc=1172379
A node is an IP address whereas a pool member is an IP address and port (with an association to the pool name). Normally, you create a monitor for a pool member without an alias IP or port and the monitor inherits the pool member IP and port. A service level monitor without a hardcoded port can't be associated with a node as there isn't a way for the monitoring daemon to know what port to perform the check on. If you create a monitor with a hardcoded alias port, then you can apply it to a node (an IP address).
The short answer is that you generally want to associate service monitors with a pool. If you need to create dependencies between multiple ports or multiple IP:ports you can use the alias IP and/or port definitions on the monitor to do so.
Hope this is clear? If not, can you describe what you're trying to implement in more detail?
Aaron