MongoDB sharding and zone tag that has a range of 1 key value
Your understanding is correct: a chunk can only be associated with a single shard. A chunk range representing a single shard key value will be an indivisible chunk, so assigning this range to multiple shards will not result in data distribution across multiple shards.

If the data in this chunk eventually exceeds the chunk size limit (64 megabytes by default), the chunk will be flagged as a jumbo chunk and will be ignored for future migration activity.

Shard keys with limited cardinality (or outliers with high frequency) should generally be avoided as they will lead to data imbalance and performance challenges.