Hey,
Like mentioned before, there are multiple ways to achieve the same result. There is not really a good/bad way in my opinion. I've added an example below to make it more tangible:
Here you can see that I have a separate custom entity called 'systems in place' (N:N).(not directly related to a contact). This entity is used to track what software solutions a company currently has and that we integrate with. So in the above example I first specify the 'systems' i want to filter (Microsoft POS). This is linked to the account in the second step (you can only select related entities). I add an additional filter to the account entity to only segment accounts with Microsoft POS as a base system and that are based in the United States of America (this is again a custom entity with a N:1 relationship to select the country from a list). However, my segment is still not finished, because I have to target contacts. So the final step is to somehow link my contact. In this case I do this by selecting all contact that belong to accounts with the above criteria.
I can achieve the same result by starting form the contact and then start selecting related entities:
(I prefer the fist method, because I think it looks cleaner and our users seem to adopt this better.)
If your entity isn't available for selection you need to make sure that the entity has relevance search turned on and is synced with the marketing application:
1: settings --> dataset configuration --> select the entities to sync
