Originally Posted by phlwookie

Been a while since I precleared in Ireland on returning to the US, but for other countries with preclearance, such as Bermuda which I flew a few weeks ago, your arrival is no different than any other domestic flight. Someone will correct me if this is different for the PHL arrivals from Dublin and Shannon.

Once behind security at PHL, you can walk freely between Terminals A (East and West) and E. If going to F, you need to take a bus from A1 or C16 to avoid reclearing security - that is the only "isolated" terminal behind security.

Immigration is where the entry of your person is either permitted or denied on the basis of your citizenship, the validity of your passport, any applicable visas, etc. and to ensure your destination country doesn't want to block your entry due to security or political reasons. Airlines validate that you have the right paperwork on checkin to minimize the risk that you get refused entry as they need to foot that bill.

Customs is where the destination country checks to ensure you're not bringing unwanted items into the country (e.g., fruits with insects that could destroy local crops) and if you're bringing taxable items in, to apply that "duty" (hence the term "duty free" of some stores in international terminals). Note that just because you can bring in some of an item duty-free doesn't mean you can bring in an infinite amount - a US citizen returning home with 25 bottles of French wine for example should expect to pay a duty on most of them. What that is depends on what it is and how much you're bringing, but this is usually easily found on the Internet so you can prepare for it or adjust what and how much you bring. This is enforced by a self-declaration form given to a customs officer, and random inspections of luggage to ensure the form was filled out correctly.

Hope this helps.