How do you define Variants?

Now that I've added a Variants section to my checklist page, I think I need to make sure that *all* the variants are listed there! My own collection isn't big enough for me to collect variants, so I'm looking for feedback from other collectors about the best way to show the variants. So to start with... how do you determine what is or isn't a variant?

One person left feedback asking for Batman-52 "no eyeballs" Thomas to be included as a variant. Is that a proper variant or just a factory error?

I remember seeing a post about Skarloey with a taller smokestack. That seems like a variant to me, but does that exist for every version of Skarloey or only some? Assuming we can find out which Skarloeys have each smokestack, what is the best way to present that on the checklist?

Anything else I'm missing?

It's a tough call to say what is truly a variant. In my opinion you could say the following are variants:
Weighted trains/unweighted trains
Different names on the bottoms of the trains
Paint or appearance differences such as in the case of Monster Mash Percy or Cyborg Spencer.
In the case of Batman 52 with and without eyeballs as a variant. As the name on the bottom of the trains are different for the one with eyeballs and with the one without. The train without painted eyeballs wasn't a one-off as there seemed to be about a 50/50 ratio of them out there. I think they were released in different regions. Ie without eyeballs in Australia and with Eyeballs in Canada. Just as an example...

I've been working on a list based on what I've found so far. Just for another example. 2017 wave 1 trains come in both weighted and unweighted. I have 2017 wave 1 tootsie sets sets both weighted and unweighted. I guess it really depends if variants are something you want or even care about. I personally enjoy the challenge of finding the differences. I still need to figure out if I have the Skarloey variant or not..