Currently the definition of a glitch given in the rules is very ambiguous
⩾Glitches that occur unintentionally and have no major impact on the run are allowed.
⩾Due to being commonly accepted by the playerbase (as well as the devs themselves), the confirmation by the developers that it will not be removed and occuring very commonly unintentionally, the suspension glitch IS allowed, despite the name.
There are also mechanics in the game that some mods consider to be glitches (even though they are at best exploits) those include transparent solids (like net blocks) and seat clipping (this is clearly what glitchless means, but the definition is vague enough that it can be interpreted as allowed.) I would like a more comprehensive list on what is actually considered a glitch and is not allowed in the glitchless category, instead of the currently incredibly broad definition.
My general understanding of the term "glitch" is anything that's unnatural behaivour, some kind of bug in the code. Where as an exploit is something that the devs overlooked. However i could be argued that the seat clipping through the ceiling is an exploit, it is considered a glitch for the glitchless category to make it actually different from the glitched category.