Since the ruleset is getting a bit crowded and some of those rules are not necessarily obvious as to why they exist here's an explanation for most of those as well as a few other things.
> IGT instead of RTA or LRT
The game is primarily timed through IGT because it's consistent across all 3 main platforms (PC/N64/PS1) and is also available on one of the portable ports (GBA).
> Requiring to include the end of world timer
All 3 platforms have an extremely rare glitch where the end of game timer can show 00:00, obviously that cannot be accepted as an actual time so in that eventuality you still have the possibility to manually add every end of world times to get you actual ingame time.
> Demo Mode Glitch?
First discovered by Ivanzypher then researched more in-depth by thestrangepie it's a simple glitch that allows the N64 version to play normally during the demo mode thus allowing you to start the game on latter worlds right away.It can be manually triggered by pressing A to start a game during the fade-out to demo mode.
All those reasons is why it has it's own category (Any% because it's the fastest way to go from starting the game to the end credits as of right now) and a separate category in the levels leaderboard.
> ILs allowing starts from saves but not level select
Saves allow you to retain powerups whereas level select doesn't and only allows for fresh starts, that coupled with allowing ILs to be submitted from full runs makes submitting ILs from a fresh start with no powerup meaningless as you're at the mercy of RNG (powerup drops are based on a per board table that drops in a random order if you hit a mothership that will spawn once you ave killed a certain amount of aliens onscreen provided there's not another one already present in which case it will be queued, but said queue will be discarded if it is supposed to spawn as the last non projectile entity in a board) until you get the right powerups which translates to up to two minute of handicap on certain worlds.
If need be a separate set of categories will be made under the condition that someone has a set of runs to submit for it before the category is made before it's made
> Differences between difficulty levels
Normal has the slowest movement speed for every entity and has the least aggressive AI for aliens although bosses are unaffected, it also stops at the 8th world.
Expert has slightly higher movement speed for every entity and a more aggressive AI for regular aliens, it also allows you to play the final two worlds of the game.
Maniacal has 200% speed compared to expert for your tank and your projectile, regular aliens are also faster than expert and their AI becomes overly aggressive.
> Differences between platforms
First off both portable ports are different games although the GB port has nothing in common with the others outside of being from the 1999 series of Space Invader ports from Activision whereas the GBA version tries to retain most of the content of the main version with some fairly big changes, I won't go more in-depth regards those since they're in their own separate categories.
Then comes the PS1 and PC version they're extremely similar with two main differences, the PS1 has very long load times and a fair bit of slowdown throughout the game making it the easiest to run, both the PS1 and PC version have the same tricks and glitches available (no glitches are actually useful in a speedrun as of right now) with one exception as the PC version will crash if you kill a large amount of enemies simultaneously (which thankfully is not actually that frequent and will only be an actual problem in two specific boards)
N64 on the other hand has some fairly big differences, it spawns entities models in each boards faster than the other versions although the hitboxes themselves are present at the same time.
As of right now only 4 of those are known to exist and only one of those being beneficial in a speedrun.
"Boss pseudo softlock", sometimes the result screen will not pop up immediately after killing a boss and will sometimes take up to several minute to come, it's unknown if the IGT is still counting while this is the case though, there isn't any known specific or way to trigger or avoid it and it affect all 3 main version, it has been known to happen on the Uranus, Mars and Mercury boss particularly
"Saturn boss suicide win", if for some reason you were to kill the Saturn boss while his laser is firing the game will not advance until you suicide into it, this is extremely hard to trigger on accident and should never actually happen in a normal run since it requires you to kill the laser last which is both slower and harder to do.
"Attack speed reset", as described earlier this one only exist in the N64 version (as far a current runners could tell) and is triggered by using the Mercury special attack, easiest known way to avoid it is to simply not use that attack.
"Demo Mode glitch", specific to N64, by either pressing A to start a new game during the fade out to demo mode (or it can also apparently trigger on it's own sometimes) the game will let you start the game where the demo mode would usually take place, meaning in practice you can start the game at mars and skip 5 worlds, as discussed earlier it locks your difficulty to normal and alters a few RNG behaviors in the game "Mothership" stages in particular.