Search string:

@abc - matches notes with label abc
@!abc - matches notes without abc label (maybe not the best syntax)
@abc=true - matches notes with label abc having value true
@abc!=true
@"weird label"="weird value" - works also with whitespace inside names values
@abc and @def - matches notes with both abc and def
@abc @def - AND relation is implicit when specifying multiple labels
@abc or @def - OR relation
@abc<=5 - numerical comparison (also >, >=, <).
some search string @abc @def - combination of fulltext and label search - both of them need to match (OR not supported)
@abc @def some search string - same combination