Tags are used to conditionally include music expressions in multiple scores prepared from the same source text.
partCombine is used to merge separate parts onto a single staff. These two features interact in ways I found surprising.
There are musical expressions that I want printed
only
in the full score. I preface these with \tag #'score. Other expressions I want printed
only
in the parts. I preface these with \tag #'parts.
The full score contains many staffs in parallel, all of which are enclosed in a << >> pair. Before the first <<, I put \keepWithTag #'score. This mostly has the expected result: it prints everything that is either untagged or tagged score, and omits everything tagged parts.
The exception is...any pair of instruments for which partCombine was used. For instance, the piece has a pair of clarinets. In the score, I want the clarinets on one staff. In the parts, I want the clarinets on separate staves. This is precisely the situation partCombine is meant to handle.
But the single \keepWithTag #'score
before the first << does not work for staves where partCombine was used. For only those staves, I need to re-state
\keepWithTag #'score before each of the separate music expressions that is input to partCombine.
I was able to discover this by trial and error, but there probably ought to be a warning about it in the manual.