Marking tiers are listed as follows on the newspost they were introduced in-
Tier 0 - common markings, such as ones found in NCLs, rolled males and Oasis
Tier 1 - custom markings, such as ones found in Marking Applicator and Oasis
Tier 2 - raffle markings - these are breed only
Tier 3 - event markings - these are obtainable via special events such as monthly NCLs, event applicators, NCL exclusive etc
Tier 4 - rosette markings - these have unique breeding patterns
Tier 5 - these are disabled from Random Marking Applicator due to unique raffle applicators, event applicators that aren't unlocked yet, mutation markings and so on.
Tier 6 - these are RMA exclusive
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So from what I understand-

Tier 5 are hybrid and mutation exclusive marks, as well as markings that are not yet available in game but are still view-able via wardrobe, thus are not applicable via RMA.

Tier 3 are event exclusive markings, event NCL(aka piety markings), event applicators(gotten in shops or via crafting during their given events).

Tier 2 markings are raffle and completely breed only, cannot be applied unless you get really lucky with a Random Marking Applicator.

There are many markings that are listed in the wrong tiers/have yet to be fixed and placed in their respective tiers, here are some that myself and a couple of friends found when discussing this matter;

Ruffian marks are tier 5 still after over a year of having them and yet Scoundrel is listed as tier 3 already. Both of these marking sets are completely breed only, and are also available via Raffle lionesses year round. These should both probably be Tier 2 since they are not available via Piety in November as most other event exclusive markings are.

Celestial raffle markings are also completely BO as they too are not available via Piety in November's event. They should likely be tier 2 as well, and are even described as Celestial raffle marks.

A friend also pointed out that the event NCL markings in colors Marigold, Copal, and Clay are not available via Piety either, thus should also be Tier 2 since they are strictly breed only, despite the fact that they are 'event ncl'.

Hyena stripes, Angelic, and ember markings are tier 5 even though they are event applicators and should be in tier 3.
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Tiers were added to prevent confusion and help players find markings easily, but with these breed only marks and non breed only marks being bunched into one tier, as well as applicator markings being placed in the wrong tier, it's confusing and misleading in many regards.

Not going to open up a can of worms, but in regards to the previous discussion of Celestial raffle markings becoming applicator markings, making them just flat out tier 2 could actually help with that problem. This would allow them to be used more on raffle ladies, as well as if they really don't want them on Celestial raffle lionesses they could just make em strictly raffle lioness marks! There are plenty of rare slots for all three colors(Celestial, Sidereal, Skyward) so if anything it'd be an improvement!

If you don't support, have questions, or have any ideas/new information please lemme know and I'll update the topic as y'all post!