Perhaps I have found a solution to a problem once been discussed on Ufopaedia. The problem is of a big importance for the game.
UFO / TFTD game researchers found alien appearence ratios in the game exe file. It appeared that also the Alien Retaliation mission has such ratios set. On the other side, it was said that alien attacks on XCom bases are always responses to XCom activity, occurring with some chance, and it is always the XCom-suffering alien race which responds - so the appearence ratios in the game seemed to be of no use. They were said to be superfluous (
https://www.ufopaedia.org/index.php?title=Alien_Appearance_Ratios: "All Retaliation rows are unused. Retaliation missions are generated in response to shooting down enemy craft, with a set probability that is scaled by the difficulty level multiplier.").
It was also observed that (during TFTD playing) Tasoths were able to attack a ship route as early as in April 2040 even if the first appearance of that race should be in June 2040 (see Ufopaedia:
https://www.ufopaedia.org/index.php?title=Alien_Appearance_Ratios_(TFTD)). Contrary to the popular opinion, no flying/floating USO recording used to be done just before the attack. It seemed that the aliens emerged from the deep, not from a USO.
Similarly, it was observed in UFO:EU that some terror attacks may happen very early in the campaign, even in the first week, and no preceding Terror Ship flights were observed even if they should be (e.g. when the terrorized city was not too far from the XCom base).
I think I have solved all the above problems. Namely, according to my observations, there are TWO DIFFERENT general Alien activity types. The first type are the well-known alien
missions, each containing several phases of scheduled UFO/USO appearances, and chosen by the game engine in accordance with the alien appearance ratios. The other type are aliens
single attacks that are not scheduled, not in accordance with the ratios, and not consisting of any phases (hence "single").
So, it seems that the aliens take a part in both (a) missions and (b) single attacks which are not part of any mission (and thus are not scheduled). Single attacks mean, among others, city terror attacks (in TFTD: shipping route, port and island terror attacks), not recorded before, not preceded by an appearance of any UFO/USO (the aliens attack immediately) and not in agreement with the appearance tables. Hence Ethereals (UFO:EU) or Tasoths (TFTD) may appear as early as in April, hence aliens may terrorize land as early as in the first days of the campaign, without smaller UFOs preceding the attack.
Single terror attacks are frequent in first months, then (exactly when?) they seem to cease. Aliens can still terrorize cities (UFO) or ships, ports and islands (TFTD) but only (mainly?) during their Alien Terror / Alien Surface Attacks missions rather than single attacks. Of course - if such a type of mission has been chosen for a current month. It is also quite probable that shooting down UFOs/USOs during first phases of the mission can stop it, and finally no terror will occur. Or (which one is true?): it will prevent aliens from knowing the localization of the city (port, island, ship route). As the result, the final UFO/USO (Terror Ship in UFO:EU and Battleship in TFTD) will appear but not find its target.
As it seems to me, a similar dual mechanism lies behind alien attacks on XCom bases. Some of the attacks are a result of an alien response to shooting down a UFO / downing a USO (with a chance of occurrence which depends on the game difficulty level). Contrary to single terror attacks, a Battleship (UFO) / Dreadnought (TFTD) always appears then before the attack. The aliens know exactly where the XCom base is located, so nothing can help XCom to avoid the battle, and the UFO/USO moves straight to the base. Appearance ratios are of no use then - it is the same race which responds, as the race whose UFO/USO has been downed by XCom.
These single attacks in response to UFO/USO downing seem to have nothing to do with Alien Retaliation / Floating Base Attack missions. Like other of the 7 mission types altogether, Alien Retaliations / Floating Base Attacks are scheduled and consist of several phases. Such a mission must be chosen for the current month (together with another mission - two missions start a month). It is quite probable (but not confirmed!) that the mission region (chosen at the beginning of a new month) must contain an existing XCom base. Smaller UFOs/USOs happening on the first phases of the mission are scouts which help aliens to locate the base (otherwise unknown, unlike during a single attack). Shooting them down can also stop the occurring of the further phases of the mission, and as a result, no Battleships / Dreadnoughts will appear finally. Alien Appearence Ratios are applied, contrary to what is said in Ufopaedia (so the data are not superfluous). It is also only a chance that the aliens will find the XCom base location (unlike single attacks when the Aliens always know where the base is located, without a preceding reconnaissance).
In UFO and TFTD the aliens start two missions each month. It is known nothing about single attacks: what is the exact number of them, or even if they depends on time at all (perhaps there are moths with no single attacks, and months with several single attacks). However, observations suggest that they stop at a certain moment of the campaign (or become very rare). Anyway, multi-phase missions dominate over single attacks (if any still possible) in later months of the game.
I would like to know if my observations (as it's been seen, partially not in accordance with Ufopaedia) are correct, and if not, how to explain the observed game behaviour.
My second question is how it all works in OXC. Are multi-phase missions and single attacks distinguished? Are the aliens able to terrorize a city as early as, say, on 1st January 1999? Does the number of single attacks (those terror ones and on XCom bases) depend on the current month, and how (e.g. two attack a month, together with two missions starting)? Is there a possibility e.g. for Ethereals to appear as early as in the first month of the campaign (during a single, non-scheduled, city terror attack)? And if appearance ratios / rules exist in the game, what are they like? The above mentioned April Tasoths (and also probably April Ethereals) suggest that they should not be the same appearance ratios as in alien missions.
In the original game, two new Alien missions start each month. Is it hardcoded in OXC, or can be modded? And: is it possible to change the rule of starting alien missions (in a game mod, to make the campaign different that in the original game) that they would start in a day chosen by chance and not the first day of each month?
