A unit with sites doesn't have a direct input any more. All incoming links lead to different sites, where the arriving weighted output signals of preceding units are processed with different user-definable site functions (see picture ). The result of the site function is represented by the site value. The activation function then takes this value of each site as network input.
The SNNS simulator does not allow multiple connections from a unit to the same input port of a target unit. Connections to different sites of the same target units are allowed. Similarly, multiple connections from one unit to different input sites of itself are allowed as well.