Match settings determine which units are ‘allowed’ in a match and optionally filter units in search results. (See Search)
Units and equipment that conflict within the match settings will be visually flagged, but still selectable and deployable.
The 🚫’s indicate the Mad Cat and the ARC-4M are not permitted under these match settings — The Mad Cat is not available in the region (Inner Sphere) at that time (3030) and the ARC-4M is available nowhere as it is not introduced until 3049. (v0.17.3)
Region includes Inner Sphere (the great houses and near periphery) and Clan Space (think anything beyond the Caliban Nebula-ish and maybe the exodus road). This is not an official division but is hopefully intuitive. I'm afraid there's no plan to make it more granular as part of some kind of faction-based filtering (which is prohibitively difficult even if I was interested). It's there to work in conjunction with the year option...
Year allows filtering out units that don't yet exist but it's also used in combination with region to filter units that don't exist in the selected region at that time.
Example: If we're in the Inner Sphere in 3027 then half of the Mad Cats shown below do indeed exist but are only available in Clan Space.
(v0.7)
<aside> 👉
Search results table columns have changed some since these features landed in v0.7
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Example 2: If we were are in Clan Space in 3027 the Inner Sphere's Hatchetman is unavailable.
(v0.7)
What may not be super clear in the UI right now is that the "region" column in the search results and roster means "region availability in the selected match year".
Note that there's currently no notion of tech regression, so, e.g. LosTech / Star League units are still going to show up in the Inner Sphere in 3015. If you'd like a 'level 1' / ‘intro-tech’ filter for this era the current solution is the rules item (more on this later).