if they are direct links to specific items of interest. Generic links to other forums are not allowed.

Example of not allowed: "check out this great forum at www.greatforum.com"

example that would be ok: "to learn more about how to install the tires on this truck see this thread www.greatforum.com/specificthread.html which explains clearly how it is done"

RCU is about sharing information. It is not about promoting shamelessly other sites for the sake of promoting them. Thus direct links to specific items of intererst are fine while outright plugs are not.