Around early summer each year, Albus Dumbledore, like the headmasters before him, would go to the tower where the Quill of Acceptance and the Book of Admittance were kept, use Geminio to copy that year's list of new students, and then hand it to Minerva. Minerva would send out acceptance letters according to the list.
The addresses on the letters were written very strangely. Severus's letter was addressed to the old locust tree where he and Lily often went, while Pomona's was addressed to the seashore where she often stayed. The sea breeze would set the shell wind chimes in the cottage ringing, and there were no other landmarks nearby.
Although owls were responsible for delivering the letters, and they could always find wizards, they were not the ones who wrote those addresses.
The Ministry of Magic used the Trace to monitor young wizards and determine whether they had improperly used magic during the holidays. When someone broke the rules, the Ministry would also send a letter, with the address written in emerald-green potion.
When young wizards accepted an invitation from a magical school, took the train from Platform Nine and Three-Quarters to Hogsmeade, and then travelled by boat to the school, the first thing they faced was the Sorting Ceremony.
Minerva would read out names from the list. The young wizards who heard their names did not need to answer "Present"; they simply sat in the chair and put on the Sorting Hat. This did not merely represent their House assignment. It also represented "signing in." From that moment onward, they had to obey Hogwarts's school rules and the International Statute of Secrecy.
They could also choose not to "sign in." If they were educated at home, there would be no Trace monitoring them, so they would not need