That would be your suffix. Those +4 numbers help with routing to your neighborhood. If you include the +4 characters it helps sorting and delivery. Hope this helps.
It depends upon the cities. I picked up a part time job this Christmas moving mail from the distribution center in Rancho Bernardo. In San Diego county we have 40 or 50 individual post offices. The company I work for delivers to the 3 in Oceanside and 4th if you count Camp Pendleton. O'side has another office that we don't deliver to. We deliver to 2 in Carlsbad. Theirs at least 1 in Encinitas and 1 more in Cardiff by the Sea. Each of these post offices has a different or multiple Zip Codes they deliver to.
It depends on where you are located.
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