Set priority in sort criteria
I don't know if it's possible ?
Hmm I don't think you can do that using only the Sort component, but there is a new index feature called "ranking functions" that I think you could use to achieve the same result. Ranking functions are different from query ranking expressions (or QRE) by the way.
A ranking function works by computing a expression for each matching result and adding the result of the function to the score of this document. So if you have a numeric custom field you can simulate a sort on that field by putting this in your advanced query expression:
$qrf(expression: '@myfield * 1000')
$qrf is the syntax used to define a query ranking function. If you always want to sort on the field just put that in the filter your Tab component, otherwise you'll need some custom JS to inject the expression in the buildingQuery event when appropriate.
This works because 1000 is a high enough value that it overrides all the combined factors coming from the stock relevancy engine. So the results with a higher value for @myfield will appear first, and those with the same value be sorted against each other using the stock relevancy score.
I do think we should have a more direct way to get this result, maybe by introducing a new kind of sort order. But in the meanwhile this might work for you.
As I mentioned, it's a a very recent features, and depending on the version you're using it might not be available. Also I can't yet find much public documentation about it (I'll see with the right person), but our internal wiki mentions that the field you're "sorting" on should be in the ComputedFieldsInMemory list from the index settings, otherwise performance might be pretty bad.
I hope this helps :)