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The optimiser normally creates a frequency histogram if the number of distinct values in a column is less than the number of histogram buckets you specify.

This is where the histogram concentrates on only the popular values.
The less popular values are considered insignificant.
Less popular values are therefore grouped together in a single bucket.

As the name suggests, a combination of Height Balanced and Frequency histograms.
12c tends to use Hybrid Histograms instead of Height Balanced.
The histogram will store the amount of times the highest value is represented in the bucket.

Oracle Base Article