IFID stands for Interactive Fiction IDentifier. An IFID is a unique serial number that's assigned to each work of Interactive Fiction, so that players, authors, and archivists have a universal, unambiguous way of referring to a game. It's the same idea as the ISBN system that publishers use for books. The IFID system was created by the Treaty of Babel in 2006.
For a new work of IF that you haven't published before, you should create an IFID for it using your development system's conventions. For example, TADS Workbench and the Inform 7 IDE have built-in tools that assign an IFID automatically when you create a new game. You can also create an IFID on-line using the tads.org IFID Generator (you can use this tool for any system, TADS or not).
For a previously published game, use an IFID extraction tool to find the IFID. The Treaty of Babel web site has a free tool that you can run almost anywhere. If you're a TADS user on Windows, you can use the built-in Workbench command "Tools/Read IFID From...", which will tell you the IFID for any game, even non-TADS games.
Every IF game ever published has an IFID, whether the author assigned one to it or not. The IFID system was intentionally designed to include older games, by defining an implied IFID for any game that doesn't use the new Treaty of Babel conventions to embed an explicitly assigned IFID. The implied IFID for an older game is basically a "checksum" based on the binary contents of the game's executable file.
Yes. The IFID for an older game (written before the IFID system was created) is based on the binary contents of the game's published executable file. If a pre-IFID game has been released more than once - for version updates or ports, for instance - each release will have a separate IFID, because each release's binary file contents will be slightly different.
A newer game with an explicitly assigned IFID should only have one IFID for its entire life cycle, no matter how many times it's published. This is because an explicitly assigned IFID is stored directly in the game executable file, so the IFID extraction tool will find the assigned IFID and won't have to calculate an implied IFID based on the file contents. So, even when the game has been published in multiple forms, it will always have the same IFID.
All of them! IFIDs are intended to permanent, universal identifiers, so once a game is released with an IFID, that IFID forever refers to that game in all of its versions.
This means that you should generally never delete or change an IFID in a game's listing. The only exception is when an IFID in the listing is wrong - if it has a typo, or someone accidentally entered an IFID for a different game, then the erroneous entry should be corrected or deleted.
But once a game is released with a given IFID, that IFID should stay in the game's listing, even if a new release has a different IFID. In this case, you should simply add the new release's IFID to the listing, and keep the old one(s). The reason is that the old version won't just disappear from the planet just because a new version came out - the old version will still be out there in on-line archives, individual hard disks, etc. Someone who has a copy of the old version might someday want to find more information about the game. To do this, they can extract the IFID from their copy using one of the Babel tools, then they can come here and look up the IFID in our database. They'll find what they're looking for, so long as the game's listing includes all of the game's past and present IFIDs.