MB4 229b: Eruv Chatzeros (366:1-2)

Shlomo HaMelech made a gezeira against carrying from one reshut hayachid to one owned by another unless an eruv chatzerot is made.

Carrying from a house into a joint chatzer is prohibited without an eruv chatzerot. Once the eruv is made it's as if everyone lives in the one house with the eruv food and the entire chatzer is subordinate to the house.

Rashi and the Rambam's understanding of the rubric of eruv chatzerot

Making an Eruv Chatzerot in a desert or for soldiers.