There is a non standard to standard map in Athena for [score] to the UCUM standard score. But [score] isn’t a valid UCUM code. But I do see {score} which is a UCUM annotation because it’s in curly braces, but not in Athena or the vocab… so, why does the invalid one exist but the valid one doesn’t ?
The square brackets indicate a unit that is not from the canonical set (= standard system of units). But it is a true unit. Examples are international units, colony forming unit, acres, lbs and all the other fairy tale units some countries still use. 
The curly brackets indicate an annotation or a description to a unit or even nothing (the unit 1). So, {score} would mean some entity that is counted, and that you call “score”. Examples are cells, copies, heart beats, actuations, etc.
So, [score] is correct. It stands for a real unit. It is not the unit 1 with a description that is used for an enumeration of things. In other words, you are not counting scores. However, people should probably use a more specific score, but still.
We need to review the units, though. There are deviations from the above logic. The current set we have is a very early list of “good” units issued by Regenstrief 10 years ago. It’s probably not that good. Also, we want to introduce conversion factors between units and standard assignment to Measurements.