These are simply a series of numbers. We give one to each etext as it is posted, so the earliest etexts have low numbers and later etexts have higher numbers. Etext number 1 is the Declaration of Independence, the first text that Michael Hart typed in to the mainframe that he was using in 1971.
A few numbers are reserved for books that we hope to have in the PG archive someday; for example, 1984 is reserved for Orwell's classic.
When we improve an text by making some corrections, we call it a new EDITION, and it keeps the same etext number, but when we post a different VERSION of the same text, from a different paper book--like different translations of Homer's Odyssey--each new version gets a new etext number.