Carrying Within a Valid Eruv
Within a valid eruv, ordinary objects may be carried between the joined domains and through the enclosed streets. The eruv changes the status of carrying; it does not permit other melachos.
What the eruv permits
Keys, food, seforim, strollers, and other permitted objects may be moved between the joined domains and through the enclosed streets. The permission applies to hotza'ah; it does not suspend the rest of hilchos Shabbos.
Muktzeh remains muktzeh. Business activity, preparing for after Shabbos, unnecessary exertion, and carrying something whose use is itself prohibited remain governed by their ordinary halachos.
The entire journey must remain inside
Both the starting point and destination must be within the eruv, and the path between them must not cross outside its halachic boundary. A shortcut across an unenclosed street or an exit beyond the line ends the permission.
The accepted map and the instruction of the supervising Rabbonim determine the usable area. A visible wire is not enough to identify the boundary without knowing how the complete route has been established.
Permission depends on the eruv's current status
A valid eruv can later fail because of construction, weather, a broken line, or an opening in a wall. The public may carry only when the responsible supervision confirms that the route remains valid for that Shabbos.
Where no accepted eruv exists, the ordinary prohibition remains in force even if individual components resemble those used in a valid route.