Band on the Run... sorry I missed your question...
This law originated in 1987. Basically, it allows certain exisiting neighborhoods and future developments to close off the neighborhood and create limited access points (usually 1 but sometimes 2). These access points (or front gate, if you like) are intended to simply "control" vehicle access. This means that a guard is placed at the front gate and will ask everyone who doesn't have a remote (to open the gate) what they intend to do once they enter the gate (visit someone, pass out fliers, whatever). The gaurd then writes down the license plate and the visitor is free to enter since all streets in Puerto Rico are considered to be public.
However, over the years, people on the island have slowly lost sight of the intent of this law and it later became a generally accepted assumption that only residents and visitors were allowed access (aside from the obvious... trash pick-up, FedEx, UPS, etc.).
It's a little more complicated than that... but that's the gist of it...