Unique hitstun falls into the category of moves like DDD's Dthrow,Foxs Shine, or most jabs that will deal the same knockback, and the same amount of hitstun no matter the percent.
Normal hitstun is all other moves that increase in knockback as percent goes up.
That about right?
Somewhat... almost all attacks have unique hitstun, but it can only be observed at lower percents, like Falco's dthrow and some of Luigi's attacks. They have the same amount of hitstun regardless of knockback, but once knockback reaches the threshold for sending into tumble, normal hitstun is calculated.
And this feature to cancel all hitstun with airdodge or aerial on frames 13 and 25 respectively is universal to all scenarios and characters? I am hit by Snake utilt at 26% as jiggs I'll be able to air dodge on frame 13. I am hit by a ZSS's ftilt at 170% I can use any aerial at frame 25 and airdodge on frame 13? When can you jump out as sson as possible?
Yes; yes; yes; and and since jumping does not cancel tumble, you will be able to jump out as soon as your hitstun runs out normally. This is actually a decent method of figuring out when hitstun "truly" ends. Also keep in mind that by "aerial" I mean "aerial A attack," as specials cannot cancel tumble either (this is why G&W's bucket brake involves a nair followed by bucket, since nair can cancel the tumble and allow for a bucket sooner than hitstun actually ends).
I do not know about "combo", but that whole idea of canceling at specific frames really determines what is best to use for MCing. It would seem that in order for an aerial to be better than air doding, it has to be activated on frame 25 and end soon than air dodge being activated on frame 13. Which most air dodges are like 40frames and using air dodge or Shieks' 28 frame Fair end around the same point frame 53. The benefit to aerials is that they can be fast fallen which reduces vertical. So for shiek, FFing Fair is the best MCing method? Is that correct?
Yes, that is the best momentum cancel for Sheik. Air-dodging is faster (invulnerability on frame 4, meaning you can't get hit on frame 17, and no aerials have negative startup), but also takes enough time that you have to keep in mind that it's punishable simply by staying right next to you until your air-dodge runs out (it has cooldown).
So if I want to get out of a "combo" I should airdodge rather than use sheik's nair, which is just as fast?
See my above response. Although Sheik's nair hits later than you would gain invulnerability from an air-dodge, the air-dodge is significantly easier to punish since they only have to follow your (now unable to change momentum) character until the air-dodge is done, while punishing an aerial would involve either hitting you before your aerial comes out (which would whiff against an air-dodge), or stopping just short of your aerial's range (which would limit options for punishing an air-dodge but still allow for it).
Typically breaking out with a fast aerial is better than air-dodging, but if they keep going for the "I can out-prioritize you" approach to juggling, then the air-dodge would clearly win out. Escape options are widely-encompassing, but still somewhat situational. Learn the situations to minimize damage.
