There's a mouthful.
Anyway, here's the background:
I know that pulling criteria from different criteria sets in a single list can sometimes have unexpected effects for users. If you pull, say, "Performance Season" from one criteria set and "Price Type" from another, then generally you will get people who have bought tickets to that Season AND have, at some point (not necessarily in that Season), bought a ticket with the chosen Price Type. But if you pull "Performance Season" and "Price Type" from the same criteria set, then you will get people who bought a ticket with the chosen Price Type in the Season in question.
What I don't know is how it works on the back end. Is it based on the View the criteria are looking at (that is, all the selects for the same View get combined) or is it based on the Criteria Set (that is to say the Keyword Category)?
What I'm trying to decide is how to deal with a request to extend the criteria available within a Criteria Set: Should I add a new keyword definition using the same view (if possible) to a defined as local Keyword Category, or does it need to be the same Keyword Category? And in the latter case, is it okay to extend built-in Criteria Sets, or should I create a new one that has all of the definitions we need?
Criteria in a criteria set are combined based on the the Detail_Tbl of criteria, i.e. if two criteria are pointed to the same table or view then they will be combined to look for a single row in that table or view that meets both of the criteria (unless you use HAS as the operator). When we train on how to figure out how two criteria will work together, we say to open the criteria details and play the match game; if the Detail_Tbl value is the same then they are looking at the same place, and will be combined to look for a single row in that place. One nuance here to keep in mind is that if one criterion is pointed to the table and another is pointed to a view of that same table, those are not the same thing, so they will not be combined. So just be careful working with views and tables.
Keyword category does not matter, though I would recommend putting custom criteria in the same category as other standard criteria that look at the same place just for keeping things organized purposes. I recommend distinguishing your custom criteria from standard criteria with a naming convention, like a prefix, rather than putting them in a unique category.
We have some newer training videos that explain how criteria work together. You can find them here:
https://www.tessituranetwork.com/Support/training/Intro-Courses/Lists-and-Output
Awesome, thank you Kevin Sheehan, and thanks Michael Flaherty-Wilcox!